I have done a search and not really found concrete guidelines. I'd love to hear your advice! Any specific brands/sources are much appreciated.

The "Woolies" and "Shadowplay" lines by Maywood I think would be 2 examples of higher quality flannel, in my opinion.

I look very closely at the actual weave. Some flannels are really not very loosley woven. Look closley at the weave are the weaving fibers close or is it the fuzzy "flannel" creating fill in between the weave. Close weaves are a sign of a higher quality flannel.
Take note of how think it feels in your hand , better flannels are noteably thicker ( partially because of a tighter weave).You can also hold it up to the light , a good flannel will have a minimal amount of light that passes through.

DO note that even some of the best manufactures will tell you that flannel shrinks , and shrinks much more than regular quilters cottons. One manufacture told be that 6 percent shrinkage is not unusual for flannel.

Six percent of a yard (36 inches) is about 2-1/4 inches. Six percent of 44 inches is almost three inches.

I would agree with the tighter weave, and no 'fluff,' brushed but not fluffy! Yes, to the shrinkage also, between 3% and 5% is normal, the more you use this fabric you will gain knowledge as to what is considered a good or bad flannel. I do not know American brand names to advise you but you can try washing a piece a few times, you will soon see if it hold up.

Gal

benartex flannels are also wonderful flannels.
one thing to look for would be if it is double sided flannel or just single sided...the double is 'soft' - napped on both sides. if it is just 'soft' on one side it is a much looser weave, and lighter weight flannel. single sided flannel can be as thin (or thinner) as regular cottons/shirtings. double sided flannel is nice and thick/weighty. no see through.

Thanks for the tips!!