Condom broke!?!?

Okay like today i was doing it with my so and like he couldnt find condoms that fitted and so he just brought a regular one and when it went in after a while of playing ye finally came but when he went to pull out only his penis came out and the condom and semen was left...naturally i was scared af so i just went straight to the chemist to get the pill and got it but now im acared about how will i know its worked!?!? Help please