If you find the number on the rod, you are good to go...If not, I would take it to a fly shop...You could take it to Bass Pro, if one is near or do a search for Scientific Angler, then you can search for their nearest shop in your area...This web site is also a good resource for knots, how lines are classified, why you need the proper weight line, etc...Anyone with good experience with a flyrod can tell you about what weight it is by trying them...If you live near a flyfisherman he could swap his reel and line and try to see if his line was in the right weight class...In addition most flyrods will function ok with a line size above or below what they are rated for....I would guess its a 6 weight, if he fished for panfish with....
You are going to have a bunch of fun catching panfish on a flyrod, that and squirrel hunting with a .22 are two of my favorite pastimes....
I would buy the best line you can afford and would also suggest a weight forward line...Do a little reasearch and enjoy...Good Luck