Good question.
I would look for copyright notices on software code, but there is so much freely available software out in the public domain, that even if somebody has one you can purchase, there should be ten more available for free.
What I would mostly worry about, is using somebodies graphics, as that is easier to track down.
We had somebody do that with cyberslice.com when we started out, and it took a while to convince them it was our graphics (because their designer said he did the graphics and was adamant about that, and the owner of the other company believed him).
Of course, I doubt you will have to worry about that angle
Most of the things (I suspect) that you will want to use, are "normal" code that most everybody uses to do specific features.
-Scott