dude , park it up on level land , release hand brake and leave out of gear.
chock the wheels if it makes you feel better.
wiggle the prop. it will turn a quarter turn.
now push it up and down and side to side at both ends where the universal joint it , there should be no movement what so ever, hold the flange on the diff then try to rotate the prop , it should be solid , repeat on gear box end.
you are trying to accertain wether or not the two universal joints , one at either end , are worn. any play in these joints indicate a worn uj and it will need replacing.
in the middle of the prop is a sliding joint , grab both ends either side of the joint and try to rotate in different directions , there should be no play here either , wear here will mean a new prop.
only use hardy spicer props and uj's. they are made by gkn driveline and anything less is just crap. these are oem parts and not crappy pattern parts.
also grease the uj's and the sliding joint regularly with a decent grease.
i use this on my wheel bearings and props. i receantly stripped my rear hubs to replace the brakes and after a year of abuse the grease still looked as good as new.
Silkolene Pro RG2 Motocross