it depends on how you your relationship ended.
i'd like to share these two instances:
1st ex, my gf for 3 years, we parted our ways, knowing that it was for the better, we were young then and can't compromise on our differences. so when we crossed paths nearly a year after our breakup, i saw her with her new bf and i was just like "hey! how are you?" and she just smiled and introduced me to her bf, there was no awkward moment and i even felt glad seeing how happy they were. although there was a part saying "sayang".
2nd ex, that (pardon for the language) bEEEEEACCCCh. was my gf for only a year because i caught her flirting with another dude. trying to impress on me that i barely had time for her so she starting chatting this guy on fb. even told me that she didn't "like" the guy, that she was only attached to him. so i said what the fook? how can a girl have sex with you during the night, then go on a date with another guy the next morning and hold hands during dates? telling me that she didn't even like the guy? the lying beeeeaaach. i tried to let it pass and forgive her but this beeeeaacch just won't stop flirtiing with the guy so we broke up, even blaming me because "i don't love her enough because can't understand why she's being such a beeeeach and i cant forgive her" lol. so a few months later, i saw her walking with yet ANOTHER guy and i was like "adrienne, i should have left you loooooong before you used me used me uuuuuuup". but the song remains true, specially the part "and i'll have the last laugh, when i see you walking with some other guy 'cause i know you're gonna end up all alon. so take these words, some good advice, and all you've done gonna come back twice". i felt quite another rage but hey, karma's a bitchier bitch right?
sorry for the long post!