I had BH from 20 weeks with #2. Can't remember when they started with #1, but pretty sure it was before 28 weeks. BH's are not usually particularly painful (a bit of discomfort and that's about it) and short. For me, it was the short-ness of the BH that helped me distinguish at times. They are usually 10 seconds or so long, rarely up to 30 seconds. 'Real' contractions are meant to be more like 1 - 2 minutes long.
BUT, then I got to full term and got these really painful 'contractions' and it was hard to tell the difference between BH & the real thing. I had a number of regular pains that lasted up to 2 minutes long and were 3 - 5 minutes apart. This happened 3 or 4 times in a week, stopping after 30 minutes the first 2 - 3 times, but the last time it lasted 2 hours and that's when I went to hospital. I wasn't in labour, but they kept me overnight anyway just to watch.
Basically, have a low threshold for calling the maternity ward. They are there to talk to and you can go & get checked out any time of the day. As Cat said if these are new & quite painful then it could be pre-term labour so please get checked out.