Dig a narrow sloping trench with one end at 12" deep and the other about 4" deep and about 24" long. Set five dimes along the trench, one set on each trench end (shallow and deep) and the other three 6" equidistant apart. Refill the dirt and ensure the dirt is packed down well. Mark the location of the trench for future reference.
Now, go get your detector and try different settings until you can obtain each dime - if possible. Try everything - think outside the box, go beyond the manual and see what works best. Make sure you are using headphones.
If you want another test, try placing a rusty iron nail on the soil surface next to a dime, say 6" apart. Try to get the dime to sound off. If it does move the nail closer. Try it with the nail parallel to the sweep and again perpendicular to the sweep. Try pushing the nail straight down or at various angles into the soil at differing distances from the coin. If you can manage one nail, try two or more, again in different configurations around the dime. Then try bottle caps and other iron junk.
With your testing, try adding discrimination to see its effect. Try all-metal and try pinpoint all-metal, if you have it. Try different coils and sweep speeds. Try when the soil is damp and again when dry.
After a dozen attempts over a dozen days you will have a pretty good idea of the capability of your detector and you'll probably know how to set-up the detector best for your kind of hunting. Though actual targets can present in many other ways that can't reliably be tested for, the mere act of fiddling with your detector for a couple of weeks will give you the knowledge and confidence you need to succeed in the field. The difference is like night and day.
-Johnnyanglo