Thanks all! I did some googling for ways to test and found that a flame test (only nitrate base negs will burn down from the top and they should have a bright yellow flame) is the only one I'd be able to do here. If I still worked at the lab, I'd be able to do the diphenylamine test (we used that all the time for several things anyway). Unfortunately, I doubt I could get the chemicals necessary now.
Once I figure out which are nitrate vs acetate, I think I will use the cooler idea. I had been thinking it was likely that all were nitrate, but now that I know that acetate was in use in the 20s, there's a good chance a bunch are that. Since he went for new things, he might have been likely to switch to acetate when it came out. Since very few look yellowed, it's tough to just guess. Only a very few have writing on the rebate as well. All are in single frame pieces and many look to have been cut slightly crooked. There is a huge variety of frame sizes and my dad has said that my great-grandfather tended to sell the last camera to buy the next newest, greatest one. The only ones of his that I have are a RB Auto-Graphlex and two Leicas. The largest negs looked like 4x5, but do not fit in 4x5 holders (even the old slotted ones). And none that I've seen so far have notch codes of any type (another way I'd be able to tell nitrate vs acetate). Was it possible he was cutting his own from larger sheets or were there film makers besides Kodak that weren't using notch codes?
If I did toss them, I'd likely burn them, not just dump them in the landfill for the reasons above.