You will get all kinds of advice on how to check your rear sag. I'm not saying I'm `THE BOY' when it comes to suspension, but , I do have some considerable years experience (more than I care to put a number to here!) so I'll give you what I consider to be the best way right here...
First, service your bike. There really is no point measuring sag when the rear end is so sticky it requires a 100lb hit to kick start the stiction. Service/clean/replace all the bearings/bushes/pivots so that every individual part of the rear suspension system will freely move as it should when tightened up to the correct torque.
When that is done, set any geometry adjustments to achieve the desired position (these make quite a difference!) and climb on the bike wearing the gear you ride in - riders will use less preload for the `nekkid' crit' than for the fully armoured up DH! Then, slide the bump stop up under the body of the shock and with a friend holding the bike upright, climb on board the bike gently, sit bolt upright on the saddle with your arms hanging at your side and your feet on the pedals. Gently dismount so that you don't compress the shock any further and the bump stop will have moved to exactly match the shock position at the sag point. The `correct' sag is one third of the whole shaft travel and should be acheived using a spring with less than the maximum recommended preload - simple. Incidentally check the front end sag in the same way, except instead of sitting back, stand on the pedals, hold the bars and lean forward in an accentuated `attack' position - again look for a third of the travel (measure the `actual' total travel - don't believe the hype!).