History and testing. The best sign is history. Has he had documented proof of having fathered children in the past. Technically this is not 100% proof, unless there is dna testing, due to the confounding factor of disputed paternity. The second best sign is a semen analysis, although that only tells you how he is "on paper" and does not guarantee that his sperm is functional. Most men with good sa's are fertile.
Male fertility. Some say your sex appeal is a culmination of your ability to father. A semen analysis is a great objective way to test.