LB63 - get yourself an indoor outdoor digital thermometer. You should probably encase the wire to the sensor in silicone caulk or epoxy to make it more water proof. Mark the cord in 6" or 1 ft increments. For 4-5 ft of water, measure the surface to bottom temperature every 6" or every 1 ft. If the surface is more than 5F cooler than the bottom you have thermal stratification. Generally for ponds if the temperature drops more than 1F per 1ft you have pretty strong thermal stratification or layering, and then deepest water (8-20ft) often will not have dissolved oxygen UNLESS the water in that 5ft depth is clear (vis >2-2.5ft). At the top of the thermocline you will see 2-3F decrease in 2 ft. If the sunlight reaches the bottom in 4-5 ft of water, the plants will make oxygen at that depth, thus aeration is not needed because oxygen is on the bottom. Reason for aeration is to put DO on the bottom sediment-surface. Note: my definition for pond stratification is NOT the standard limnology definition of thermocline which is >1 deg C decrease per 1 meter. Some authors have defined the thermocline as the depth at which the temperature changes most rapidly and the thermocline (aka metalimnion) as the entire zone over which the temperature is changing rapidly. Thus the specific definition of thermocline depth is not always the same in every body of water. The thickness of the thermocline is variable from pond to lake. It is somewhat variable and depends on conditions. It is common in ponds for the thermocline layer to extend to the bottom.
For More info read through this thread:
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=443810 
