There are two approaches approved by the ACOG. One is to test all women at 36 weeks and actively manage any GBS in labour - typical high dose IV antibiotics.
The other is to expectantly manage it. No testing at 36 weeks. GBS is a problem in premature babies (<37 weeks) with a long duration of ruptured membranes. Identify those infants and watch for signs of GBS and give anti-biotics as required (prophylactic antibiotics are also OK in these situations).
GBS testing is not routine in the UK, so I first encountered it when I was pregnant in the US. Given my obstetric history - 4 babies all born at 40 weeks, spontaneous onset of labour, quick first stage with SROM at full dilation, followed by short second stages - the risk of GBS was very low. Add to that that I am allergic to penicillin (the ab of choice), there was little need to know whether I was carrying GBS or not, as there was nothing I would do with the information.
With any intervention in pregnancy and labour, you have to apply the BRAN line of questioning:
What are the benefits of the testing or treatment?
What are the risks of the testing or treatment?
What are the alternatives?
What happens if you do nothing?