Are there any signs to tell if a man is fertile or not?

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.