Know when to try with ovulation tests, which tell you, in advance, when you'll ovulate. Also called OPKs - Ovulation Predictor Kits - they work by detecting a surge of Luteinizing Hormone (LH) which typically occurs 24 to 36 hours before you ovulate.
Ovulation Test Strips
Test strips are used by dipping the tester into a urine sample first collected in a cup. Combo-paks, which include pregnancy tests, are convenient money-savers.
Ovulation Midstream Test
Midstream tests are held in your urine stream (i.e. POAS - pee on a stick!)
Ovulation, Timing, Getting Pregnant
Having advance notice of ovulation, and with an OPK up to 36 hours notice, is advantageous because an ovulated egg only survives for about 12 to 24 hours, whereas sperm are viable for up to 5 days. So that their paths cross, you can use the window provided by the ovulation test to time TTCing. If you start TTCing as soon as possible after you get a positive OPK, you potentially have about 2.5 days of overlap (36 hours, or 1.5 days of advance notice, plus the egg's survival of 1 day). If you ovulate a little sooner, say within the 24 hours (1 day), you have about a 2 day overlap. A good amount of time for your waiting egg to encounter a mate!