I'm a 28 year old male. How do I tell if I have low testosterone?

Hypogonadism. While it is unlikely at your agr, iit is possible. Have your levels checked with a blood test. If low, there are additional blood tests that would also need to be checked.
Draw AM lab. Have your doctor draw an 8-10 am testosterone and free testosterone blood test, it's pretty simple. Ask your doctor if you have concerns or questions. It's not that common at your age, but possible.