Try going quite slow in 5th/6th (ie low revs) and flooring it - see if the revs go up but the car doesn't accellerate. If so it's your clutch.
i've done this, the car does rev up quite a bit, but then it will start to go. so it feels like the clutch slips and then gets a grip everytime. there no burning smell ever either. i'm totally miffed