If you are really worried, you could talk to your GP or CHN about it.
My DS had less than 10 words at 18 months, I brought it up with my CHN at his 18 month check. She suggested I leave it a little longer. A month or so later there was no progress so I got a referral from my GP for the Hanen - It Takes Two to Talk program at the hospital.
DS is nearl 2.5 now, and is still a bit behind but has improved alot.
The best thing I took away from the program was, when they are beginning to speak, to keep things really simple. Try not to use more than 1 or 2 words if you can. Eg, if they point to cup, using the PP's example, just say 'Cup' or 'Water' whatever you prefer. Try to just use the specific word. Sounds really simple, but it has worked wonders for us.
I don't think it hurts to look into it early, although at 15 months I wouldn't be too concerned, the things we learnt in that program were really simple, yet really effective.