How do you know if you can get pregnant?

Generally, if you are having regular periods, you most likely are ovulating, unless it is determined otherwise, and pregnancy is most likely a possibility. Even with irregular cycles, ovulation can occur, although many times not as often... thus lower the odds of pregnancy. Timing is only one factor in the process of becoming pregnant, albeit an important one.

If someone younger than about 35 has been actively trying for a year, testing will usually be suggested. This is because most couples will become pregnant within the first year of trying... however, some will take a bit longer even when all is well.
Women older than about 35 should look into testing after 6 months when unsuccessful. Anytime a women starts testing, her partner should also start testing as many times it is the male who has fertility issues... sometimes both partners do. There are numerous options to help with fertility these days.

The only way to know for sure is to either become pregnant, which is obvious, or to have some testing done to see if a fertility problem can be found.