Once all of the onot perishah have passed without menstruation, there are no further restrictions on sexual activity, excluding a woman with a veset kavua for a yom hachodesh (perishah on that date the next calendar month) or a veset haguf (perishah upon experiencing her symptom).
A woman is permitted, but not required, to check before relations to make sure she is not bleeding. If she is concerned, we recommend wiping externally with toilet paper, not immediately after urinating. If she finds nothing problematic, they can proceed with relations. If she finds a stain on the toilet paper, relations should be avoided (as a matter of prudence, although they are not technically forbidden), but she is not considered niddah.