As far as the attic is concerned, put the light on in the attic, leave it on 24/7 and position a cracker under the light, smeared with a bit of peanut butter, put it in an EXACT position, so you'll know if it's been touched, and then check it on a regular basis for a few days. If you go for a week or so, with no activity, you could possibly assume there are no interlopers.  (During that time, at any rate.)  Naturally, if it disappears, you could have several different reasons.  It could be squirrels, it could be mice, it could be chipmunks, possums or even raccoons.  Or any local animal that finds your chum tasty. For a PERMANENT detector, install a trap up there, (the HavaHart 1040 would be preferable) and just LEAVE it there, set and ready to go. Check it on a regular basis. For "bait," don't use any kind of food.   Use a fuzzy material, (cotton would be appropriate, they like to use it for nesting materials) and tie it to the treadle with dental floss or secure it with super glue.  For best results, leave the trap under a light that remains on.  A florescent light will use less electricity, generate little heat and florescent bulbs, when they remain "on" all the time, will last for years without needing replacement.  Remember that squirrels are diurnal daytime animals and will be attracted to areas that are lit.  That could give you a bit of security. Also remember that mice or other rodents will like the fuzzy material, so if it disappears, without setting off the trap, or trapping anything, mice may be the culprit. Should you discover this happening, re-bait with cotton and keep the trap CLOSED.  Mice will be able to get in and out of the trap easily and the material will still disappear. That will alert you to the presence of mice, rather than squirrels.  Naturally, it's possible to have both mice AND squirrels at the same time, so be aware of that.  And if your trap is overturned or moved around, it might be something larger than squirrels, a raccoon, possibly. Mice will generally leave plenty of droppings around the areas they frequent, so look for those signs near your chum or the trap.  Fresh droppings will be soft, older droppings will be hard.   Check your trapset occasionally, trip it a few times to be sure it's operating correctly. One enterprising customer of mine even put a baby monitor up there, he could just turn on the unit he kept downstairs to listen for the noise of the squirrel searching for a way out of the trap, rattling the cage and setting off his baby unit downstairs. For a low-tech "detector," place balls of cotton in EXACT positions so you'll know when they are disturbed by foraging animals.