Anytime you think you may be pregnant, it is best to take a test.
Tests are generally accurate 2 weeks after intercourse. The best, most accurate time to test would be after a missed period.

You would not experience pregnancy symptoms (other than a missed period) prior to about 5 weeks along.

Seeing as it's almost been 3 weeks, I would go get a test kit that includes more than one test. Test now and again next week.