As Lonnie has indicated, mumps doesn't directly lead to infertility but orchitis (i.e. the swelling of one or both testicles), which is sometimes a complication of mumps, carries a fairly small risk of fertility problems.
If you know someone who's had mumps in adulthood, he will certainly know whether or not he's had orchitis as well. It's impossible to miss; it feels like a thousand rugby players are repeatedly kicking the scrotum from in front, while a thousand demons insert red hot pokers from the rear - and that's just the start! This is written in the light of (exceedingly painful!) experience :-)
Chris