Fionas - Anney is right about the FSH level, as it rises when women get closer to M. Also, another indication might be small follicle count in the early phase of your cycle. At my clinic, they did a follicle count and gave me a rating (A=very good/high count to E=very few small follicles) - those follicles dont go on to ovulate all at once but it can be an indication of your egg quality (if you think about it, the body chooses the best of the smalls to focus its' energies on for ovulation, so if there is only a small batch, chances might be the quality isn't as good as what it might pick out of a big patch). Docs will count follicles and do FSH if you ask for an "assessment cycle" at a clinic. At my clinic, the assessment cycle cost about $250.

Best of luck with what you decide ...