The best way is to remove the rear driveline, then lay it flat on a bench...notice the "pollished" area near the seal at the slip yoke....this pollished area is the "sweet spot" where your driveline rides most of the time at normal or lifted ride highth...

To check it, move the male and female pieces back and forth it the "sweet spot" and see if its a tight fit, or if she feels a bit loose....also give her a bit of a twist back and forth to check the tightness as your slideing it through the stroke....

Some places in the stroke are or may be tighter than others...

New slip yokes have like ZERO side-to-side movement...FYI

While your at it slip back the seal, and pull the male part of the shaft out for a good look...wipe any old lube off for a good inspection of the splines...wear will be obvious here with a close clean look.

Lastly, give the Ujoints a tug or twist or 2....feel for stiffness, smothness. And side to side (rotational) movement...

Let us know how it goes and feels...