My father has swelling and pain in one leg — could it be DVT?

My dad has recently developed some swelling and aching in one of his legs, especially around the calf. He hasn’t had any obvious injury, so we’re not sure what could be causing it. I’ve read a little about Deep Vein Thrombosis (DVT), and some of the symptoms sound similar. How can we tell if it could be DVT or just a muscle problem? Should I take him to see a doctor to have it checked, and what kind of tests are usually done to confirm it?