If you have an internal or a swab at any point for anything else, they usually do a whole load of tests all from that one swab, but it's not something that is routinely tested for in UK. Sometimes it'll show up in a urine sample, but it has to be quite a severe strain of it to be present in your urine.
As for symptoms, there are none!! The only thing that can sometimes indicate GBS is a very watery, white-ish discharge, but most peole even miss that being as while pregnant we tend to have more and unusual discharge anyway, but with GBS there will be lots of it. And that's only if you get anything at all.
I'm only repeating what my MW told me though, so don't hold me to it!! Lol!! If you do go through birth without knowing you have it, the chsnces are baby will be fine and even if the GBS is passed on, it'll be a quick dose of anti-biotics in hospital and baby should be fit & well before you know it.