We're not advising that you turn into some paranoid, Instagram-trolling psycho here. That'd just be looking for a fight. However, if you happen to notice some new guy "liking" a bunch of your girl's photos, especially ones where she looks especially hot, then it may be time to wave the red flag.
Once suspicion is raised, you could always pull a CSI and do some cross-referencing. If it's not a one way street and the mystery guy has photos that she's recently liked, well, then, that's an entirely different ballgame.
And if your gal is all of a sudden taking those annoying, self-portrait, glamour shot types of Instagram pics, and then you see that mystery dude is "liking" them, let's just say we wouldn't go shopping for engagement rings. You're relationship may have an expiration date.