You can test for syphilis and herpes at 6 weeks. Syphilis is uncommon
in most developed countries (the US, UK,
Canada, most European countries, etc.). You'd get a round, painless sore at 10-90 days (average is 3 weeks), so if you don't get that, I wouldn't worry so much.
You can get a type specific IgG herpes blood test, but please note that there are a fair amount of false positives on the hsv2 IgG test.