I definitely would not admit to taking Percocet without a valid prescription at some point in the past. Opiates (the oxycodone in Percocet) are usually on the basic illicit drug piss test that they run. As far as if it will still flag, I'm not an expert.
Most pharmacies sell self test kits for like $20-40. However, I don't know just how accurate and sensitive those are. Can't hurt to drink a lot of water tomorrow. Don't overdo it on Friday with the water though.