How will I know if I have termites?

An active colony of termites has the presence of live termites, the winged variety is what you'd normally notice. Otherwise, look for mud tubes extending from the ground to your house, along the foundation walls. To be sure, crawl under the house, and poke and prod for rotted wood, and look for the presence of termites and their damage. Don't see any termites? Then you don't have any. That doesn't mean your house never did before, maybe somebody treated for them. But if you don't find any live termites, the treatment worked. If you had termites, you'd be seeing termites. There are lots of wood-boring insects that may have been there previously, but most of them won't live in dry wood. If you don't see insects themselves, I wouldn't be worried.