Trying to air test mine in the house,I get the right tone,but the cursor jumps around all over the screen.Never tried it much though,I just take it outside.I hunt about all winter,unless the ground is froze rock solid,so that doesn't bother me.
But with that being said,you still can not go by that only.Sure,everything will hit the same place when the conditions are right.But when the soil is highly mineralized,or there is alot of garbage around,they will not lock up the same place,sometimes they will not lock at all.The coins that read exactly like they air test(except when you air test with garbage,coin with a pulltab,coin with a nail,etc.)are few and far between at hunted out sites,At least around here.Most coins comes from heavy trash or just so deep that the others missed it.
The best thing to do,IMO,is to find someone with Explorer experience in your area to hunt with and have them let you check some of there coin signals when they get the iffies.At first a coin with junk sounds the same as just junk,but there will come a time you will be able to pick the good iffies from the bad iffies.You just need to dig everything for a while till you get it down.Not really many ways around that.But it will not always be like that.I only dig 1-2 nails if I go out all day.Now copper wire and brass junk,is a different story.
If I am sweeping and get say a mid tone,or a null,with a high little cutoff blip thrown in with it.I will make short little back and forth swings trying to see if I can isolate the high blip from the other signal.If I can somewhat,I will then completely circle the area with the short little strokes trying to keep the decent sounding target under the coil.If you turn your body and the decent tone in there appears to move to a different spot,that is when it is more than likely a nail.But if I can keep the target isolated in a very small circle and get it to repeat from more than one direction,that is the iffies I will dig.Sometimes you have trash so close that it is hard to get the target you want to go after to pinpoint.When that happens I will come in at it from different directions to get a pinpoint.My X pinpoint might be a v or just a 20 degree angle or 90 degree,but not an X.It is really hard for me to explain it.Where the pinpoint is on the trash signal,I will go in with the center strip of the coil at a direction that the center strip stays as far away from that target as possible.