Not generally; you normally won't be able to tell visually that
a person may have an STD.
In some cases, you might be able to recognize blisters similar
to cold sores, which can be indicative of the Herpes virus, but not
all herpes causes these blisters, and not all blisters are symptoms
of herpes. Severe STDs may cause a visible rash or an odorous
discharge, but again, these are not reliable indicators either
way.