Explain that you were checking the oil or something routine and noticed the belt and that it didn't look new. Ask them to ask the mechanic if he/she remembers (I know, it's a stretch) changing it. Point is, the mechanic will likely look at it and say "I don't remember but that sure doesn't look new or even a month old to me". Many cars it can be changed in less than 30 minutes so hopefully they'll just agree to replace it.