One way to tell for sure is to GENTLY pull on the trapdoor. If you meet a lot of resistance, the snail is alive and doesn't want to be bothered. If it pulls out limply, he's likely dead. Also a dead snail smells EXTREMELY bad. It's bad enough that if you handle it with your bare hands you will likely need to wash them twice.
Many snails go thru inactive periods. Are they producing any slime? Also, any new additions to your tank or water changes? Sometimes they are sensitive to traces of chemicals or if you've medicated your fish lately.
Your temperature is fine if it's in the 70's. My tanks are kept in the mid-70's with Endler's Livebearers & Dwarf Cories. I had problems a while back with my water clouding up a lot & lost 3 snails because of it. The fish weren't affected.
Keep an eye on them. Also, do you supplementally feed them? They can't survive on discarded fish food alone. Tempt them with food. Try Hikari (or other brands) Algae Wafers, Tropical Tablets, even a couple canned green beans are favorites of my snails. You can also buy cheap pond Koi food if you prefer. Generally snails are more active at night, but I've found mine are equally active during the day. Try to view them at different times of the day and see if they perk up. :P