For this tag to apply, it should be clear that the sex still hasn't really finished, but is merely winding down. The persons involved should still emotionally focused on each other, doing things like cuddling, holding hands, making eye contact, and similar indications of affection, possibly also (literally) sleeping together through the night. If the partners have disengaged from one another, then the sex really is over, and this tag should not be used.
Contrasted with after sex, which indicates graphic, physical evidence of prior sex, use of afterglow should be based on the apparent emotional state of those involved. Also note that, while they can coexist, these two tags tend to be mutually exclusive due to reasons of conflicting mood and style.