You are much more likely to get the truth with a simple direct question, than by beating around the bush looking for signs and hoping the subject comes up. My ring finger for instance has no tan line or indentation from a ring, because I have not worn a ring for the lat ten years, even though I have been married both happily and at times unhappily, but always faithfully to the same person for the last 13 years. I also have friends who have worn rings even when they were not married, though usually while in a relationship or down on the dating scene.
If you are not satisfied with the answer you get, then consider this: Most liars rarely tell a direct black and white lie, because it is too risky. A simple answer is most likely to be true. Liars prefer to rely on partial truths, deflection, and misleading statements. Beware if in answer to your question they dive into a long interactive story that never gets around to answering your question, but rather diverts onto another topic entirely. If this happens, don't forget your question, and ask it again. This shows that it is important to you, and that you are not easily manipulated. You may follow up with a question of why they didn't simply answer your question the first time, and if you're still interested, do some research.