|Photo not mine|
1. Look for things that do not make sense.
You should study the area before doing your survey to see what types of things you should expect to encounter. Learn about the different types of soil and rock, the known underground waterways, and things of that nature. Then, when you are taking your readings, just look for anything that does not add up, that does not make sense. You might have uncovered something unexpected, but you might be getting a faulty reading. Either way, looking into the situation more closely can help you understand what is going on.
2. Read about other surveys taken in the area.
While researching the area, it is a good idea to see if any other surveys have been done nearby in the past. Look for surveys that were carried out with similar equipment. Familiarize yourself with the results that they got. Any time that your results seem wildly different than what they found, you need to take a closer look. Again, this does not mean that the equipment is malfunctioning, but it could be an indicator that it is.
3. Always do multiple readings when you can.
There are some jobs that may not give you time to do multiple readings, but, if you do have the time, you should certainly do a number of them. If you get vastly different results every time, you know that something is not working correctly. The readings should be fairly close to the same. You can then check on the sensors and the equipment to see if you can find a problem that could be causing you to get all of these results that seem so erratic.