If you've got a centre stand or some way to support the bike so the swing arm is not taking the weight, then you can disconnect the dog bone linkages to the swing arm and easily move the swing arm through its range to check the tension at the tightest point. Same advice as Matt's above, except it is easier to disconnect the dog bone linkages than to disconnect the shock (well, on the R at least). And as Matt said, once you've done it once you know what degree of slack is required for future checks.

I've got shorter linkages (equates to greater swing arm angle and raised seat height) so doing the above was the best way to determine how much slack I need. On the side stand, it looks like the chain is overly slack but I know it's the right amount once the shock compresses.