It's difficult to know exactly when you will ovulate in advance. Usually, the time between ovulation and the first day of your period remains constant, at 12-16 days. If you have regular cycles, simply count back 14 days from the end of a cycle and you can guess that ovulation happens within a couple of days of that date - e.g. if you have 28 day cycles, you almost certainly ov around day 14, or at least between days 12 and 16. If you have 32 day cycles, it's probably around day 18.
You can buy ovulation predictor kits (OPKs) - tests that you dip in your pee, and which detect the presence of a hormone (LH) that increases 12-36 hours before ovulation. You can buy them in Boots (fairly expensive) or from eBay (much cheaper!) and other websites - try AccessDiagnostics.co.uk. If you have regular periods, you start testing a couple of days before you'd expect to see anything. If irregular, well, you get through a lot of sticks.
You can also buy the Clear Blue Easy Fertility Monitor (CBEFM) - costs around £90, detects oestrogen surge as well as LH. Good if you're very irregular and don't have a clue. Probably not worth it if you're regular.
The only way of knowing whether ovulation actually took place is by charting your temperature every day, first thing. Your temp rises on or shortly after ovulation, and stays high until your period arrives. That doesn't help you to know when you WILL ovulate, but over time, it helps you to detect a pattern (if there is one), and work out when you're likely to ovulate each month.
The cheapest and lowest-tech method is the first - have intercourse every other day for ten days around 2 weeks before you expect your period, and you should be covered.
