Spotting or period?

I'm about half way through my first placebo week, and I woke this morning with some blood in my underwear. My period isint due for an other week (6 days to be precise) and I know that spotting is common, in the first few months of being on the pill, but I guess I just wanna know, how do I know if it's my period or if it's spotting? (It's not heavy, but then again my period on the first day is never heavy)