Bike should be level (front and rear wheels level, and bike standing straight, not on side stand). Unscrew the oil gauge dipstick (lower left of bike), take it completely out, wipe it clean, re-insert it but without screwing it back in. Remove it once more to check if the level is between the two lines on the dipstcik (I like to keep it as close to the top line as possible, without actually reaching it). DonĀ“t forget to screw the dipstick back in when your all done.
I usually set up on a level surface and actually straddle the bike for this operation. That way, you can feel if the bike is perfectly straight.