Most texture artists give freely of their product and only ask that you credit them in the readme. It is a good idea to try to e-mail them and ask if you can use their work, but sometimes their e-mails don't exist anymore.
Standard textures that have been released with Retail games (CS, TFC, HL ++) are understood by the community to be used as the mapper see's fit, but it shows good form to credit the original author... if you know who that is, or at least mention that the textures used are not yours and thank whoever made them.
I was surprised by a recent e-mail asking if someone could use some sprites I produced in 1998... I had forgotten that I had developed them, but it was great to get the e-mail and even better to say... Sure, feel free.
If you don't get a reply, just use the stuff and credit the author... it is pretty well an accepted way of doing things in the mapping community.