Terry, the only thing for this is to gain experience to get a sense of too tight vs. too loose, including the swelling, which always happens. Things that might help:
1. Do a clamp rehearsal every time. Make sure there is little or no thinking or fetching of clamps during the actual glue up because you have everything ready, all the clamps close to the right openings and at hand, and know the order of things.
2. Don’t let the glue sit after application. As much as possible, you want joints to go together as soon as gluye is on them vs. putting glue on everything and then starting to assemble. Keep this in mind during rehearsal and search for a way to make sub assemblies.
3. Don’t be too fussy. Get the glue on, get the joint together. You don’t want to slop glue all over the place, but you also don’t want to slow yourself down by being too tidy
4. Make sure there is some extra room for the glue. If the volume of your tenon is exactly equal to the volume of the mortise and you pour in a glob of glue, that glue will not have anywhere to go. The tenon will slide in, the glue will pool at the bottom, and you will not be able to drive the joint home. The chamfers Paul puts on the tenon don’t merely make it easy to start the tenon. They give a place for excess glue to gather, so don’t skip them. For hand chopped mortises, I think this is rarely a problem because the bottom of the mortise is rough, but keep it in mind.
One nice thing about OBG that Peter mentions is that it is reversible. If you get in trouble and cannot assemble the joint, you can pull it apart, let it dry, adjust the joint, and try again. The glue will reactivate with moisture and heat and reapplication of more glue. PVA glue doesn’t act like that and, once on a surface, I worry about having it dry and then applying glue on top of it. But, I think Larry posted info about that being less of a worry than it might seem. I can’t remember where / when he did, though.
I’ve had a couple M&T joints that I thought were too loose before gluing, but were fine once glued up.