Agree with Janeray. If you're strumming & singing, it will sound different with a low-G than a re-entrant tuner, just because there's a lower note in there. Some people like that and some don't, but it doesn't make a difference in what songs you can play.
If you're playing melody notes, then you need to do a scan if there's any notes below C. If there are, then the song won't work, if there aren't, then you're good to go. I can think of at least a couple songs that have literally one single note in the whole song that's below middle C, and therefore you can't play on a re-entrant uke, which is a little annoying, but definitely something to be aware of.
If you don't have a low-G or for some reason you don't want to go there, you can try to transpose the song to another key where there aren't any sub- middle-C notes. Or, you can play around with substitute notes or a chord that can go there instead of the dastardly note, as there may be a clever way around it.
Good luck.