There are a number of observations that have been made over the years that seem to indicate that there are particular characteristics that "genuine" (or non-mandmade) formations appear to exhibit. Some of the observations are listed below:
In what kind of crops do they occur?Crop circles have been reported in mostly barley and wheat. But, they also appear in corn, oats, oilseed rape (canola), grass, ricefields, trees, sand, and even snow.
Where do they appear? They have appeared on almost every continent and in over 70 countries worldwide -- including the US, Canada, Australia, Brasil, Russia, India, Germany, and South Africa. There seems to be a correlation with many forming near ancient sites.
Physical changes of plants and soil - not only does the physical appearance of seem to change (it looks dehydrated), but there is change at the molecular level. Research has been conducted over a ten year period with samples and thorough laboratory testing carried out by the biophysicist Dr. Levengood, Nancy Talbott, and John Burks -- the BLT Research Team, and a small army of volunteers worldwide. Their findings include enlarged cell walls, expulsion cavities in the nodes of the plant stalks, significantly extended node lengths, and changes to the soil composition (ie. vastly higher level of magnetite concentration) from samples taken within the crop formations in contrast to the control samples taken from outside the circles.
Reported increase in crop yield - Some farmers and researchers have independently reported greater yield in the years following the appearance of formations in their fields. In 1997, Tim Carson who farms East Field (where the "DNA" formation appeared in 1996) reported to researchers that his yield was up 30-40%. The increased yield that usually only comes with annual rehybridization (done in labs) remains the same level even in up to 5th generation crops coming from crop circles, according to researcher Steve Purkaple.
Swirl in multiple directions - many formations have complex woven patterns of the swirled crops. Not only do the plants in the formation swirl clockwise or counter-clockwise, but sometimes certain sections will swirl one direction and layers on top are going another direction. They sometimes even have multiple layers swirled in different directions.
Woven stalks - Other times the stalks are actually woven like a piece of loose cloth on the floor of the formation. This has been observed by various researchers in the UK.
Selective swirling - Even though the edges are clearly defined, sometimes stalks from the outside edge are pulled into the formation. This doesn't seem to be too unusual, but actually certain stalks from the middle of a grouping of plants gets pulled in. The ones closest to the formation do not. And this occurs around the entire edge of the crop circle. Very meticulous!
Equipment malfunctions - There have been numerous reports of electronic equipment failing in crop circles and compasses spinning out of control in & over the crop circles (when flying over in aircraft). This range of equipment includes watches, mobile phones, batteries, cameras... fortunately no pacemakers yet! No explanations for these occurences, other than the indication of a strong EM field distortions.
Formations "transform" into new patterns - There have been instances where certain formations take on an entirely different appearance as they continue to grow in the field. For example, one formation got a wavy look within the center of the circle that wasn't there originally.
Unusual bending of plant stalks - It's a widely publicized fact that plants within a formation actually bend at the naturally occuring nodes 90 degrees. This can also happen in hoaxed formations where the plant is pressed to the ground and "bends" to reach towards the light. This process known as phototropism is more pronounced in younger stalks, often reaching up towards the light in 1-2 days. However, what isn't well-known is that there is more than one possible place for bent nodes. Some formations have had stalks all bent from the same node (there are up to a half dozen nodes on one plant). Some formations show bending at the knuckle closest to the ground, yet in a neighboring section they all bend two knuckles up and so on. In the line of ~24 circles in Windmill Hill in 2000, virtually all the stalks were bent 1/4" beneath the nodes, though at completely different heights (1" to 6" range) from the ground. This means that each stalk would have to be bent individually to give that effect.
Occur far from any tramlines - many formations and plain circular "grapeshots" often occur in the middle of fields far from any tramlines without any disturbance of the surrounding crop (which is detectable). Good example is the 1996 formation in Basingstoke, Hampshire where a circle with a ring was put inside the middle of an organic oilseed rape field that had no tramlines at all and the crop was 4-5 feet tall!
Formations occur inside restricted areas - numerous accounts of crop circles appearing inside military installations that are fenced off (quite securely!) from the surrounding area. Most noteably in Wiltshire along the Salisbury Plain. Are Doug and Dave in the Army?
Geophysical features - One common denominator of many of the crop formations is that they occur over underground water supplies and land situated above chalk beds, according to researcher Steve Page. Water is a good conductor of electrical current, and could possibly be channeling the electromagnetic currents of the Earth (which is what dowsers attempt to detect). This observation may have something to do with how some crop circles are formed.
Sacred geometry - Researchers have discovered layers within layers of information contained in the crop circles themselves. There are sacred ratios, such as phi, that governs the growth process of all organic life. This is an area that requires more attention and resources, as perhaps this understanding could reveal a message or at least "higher intelligence" behind these artistic patterns.
Physical side effects - Many people mention having some sort of physical reaction (positive or negative) during and after a visit to a crop formation. Side effects range from nausea, headaches, dizziness, tingling sensations, pains amd giddiness... to getting literally knocked off their feet! Sometimes the effects are felt only after leaving the formation, such as sickness or disruption to the mentrual cycle, which could be affected by the surge of energies absorbed from within the formation.
Alignment with natural features of the land - This isn't apparent from the ground, but aerial photographs have shown that often formations are imprinted on the earth in alignment with tram lines or even darkened sections on the earth. This is an impossiblity without been able to see "the big picture" before permanently laying out the design.