The way you can tell, if the tires aren't wearing oddly, is by putting it onto the machine. It doesn't take much time to check out and they get a color printout telling you exactly what's wrong, if that's the case. Of course they can fake that (getting a slip from a known-bad car, or reusing the same one), but if you need an alignment, they should show you the justification.

That said, yes, if you change the axles, you need to do an alignment afterwards. Most of the front suspension bits, if you replace them, you have to check alignment next.