How do I know if our condom has leaked?

Morning After Pill. You can try to inflate the condom to see if there's a pinpoint hole. Nevertheless if you suspect the condom leaked, broke, or was forgotten then taking the morning after pill (emergency contraception) can minimize the risk of unplanned pregnancy.
Because it broke. If condom broke and leaked its contents, it would be clearly apparent. If it wasn't visible to or your consort, then do a pregnancy test if you miss your period.