Most noticable when the car changes gears. Like from 1st to 2nd...usually shifts around 3000rpm right? And usually a quick shift. Slipping would be when the rpm goes from 3k in 1st, 3.25 while switching, and 1 second later, 2k and in 2nd gear. Basically, tranny isn't quite grabbing the next gear.
Try flooring the gas and see if the gear changes are quick. Oh yeah, I assume you're refering to automatic and not 5sp...cuz 5sp clutches slip...kinda similar.