This might sound like a stupid question but....

Okay so I'm currently on the progesterone only pill meaning I don't have regular if any periods. I have sex without a condom and he pulls out and in on the pill but I still sometimes worry about getting pregnant. But I basically don't have periods. Just occasional bleeding so how can I be sure I'm not pregnant?? Is spotting and implantation bleeding similar? Arr there any other ways i could tell whether I'm pregnant or not? This is just a general question as I'm sure I'm not pregnant now.