help! implantation bleeding vs period

How do you know the difference between getting your period and implantation bleeding?? I woke up this morning with what I thought may be my period - but the blood was a bit darker than it usually is (sorry TMI), and I put a tampon in but then about 30 minutes later it's more brown ish / blood spotty ish rather than a period bleed... but it could just be the beginning of my period?? How do I know??