Some of the free font sites out there offer information on the usage of the fonts themselves. Free Mac Fonts, IIRC, tells you by font which ones are free to use commercially and which ones are not. I used to have this same conundrum, since the maps I produced ended up in role-playing game books. In the end, I just decided to play it safe and either buy the fonts I wanted to use, or stick with fonts that came with my computer. Thankfully, there were plenty of large font collections that just cost a few bucks. Most of the fonts were junk, but the ones I did like made the purchase worth it. For me, it was all a matter of risk management. I wasn't looking to rip anybody off, so if that meant spending a few bucks to illustrate work I was getting paid for, I had no problem with that whatsoever.