You may want to start by specifying which species :). My first thought is for rabbits, and for them you know she's ready to mate when you send a buck in and she "submits," or lays low and arches upward, allowing the buck to mount. Rabbits go into heat multiple times in a day (thus the reason they're used as comparisons for certain human characteristics) and are pretty much always ready to mate. Allow the buck to "cover" her several times over the next few days, then you can pretty much be certain she's pregnant. If she took, you should be able to feel the babies in a couple of weeks, and she'll drop them in about a month (28-31 days).