How to tell the difference between an eye infection caused by a sinus infection or pink eye?

When it's caused...: ...By a sinus infection and there's redness and swelling around the eye, that's called periorbital or preseptal cellulitis. When there is no or minimal swelling, but there is redness and pain and maybe discharge, that's pinkeye. Note that you can have both at once, that periorbital cellulitis can happen without a sinus infection, and sometimes it can be hard to distinguish which came first.

It's not easy: You can't always know where the infection started. Also, some infections appear very similar to each other. More importantly, viral infections - like pink eye - do not need to be treated, while others - like bacterial eyelid infections - need medication. Knowing the origin does not necessarily help, so it is best to see your eye doctor.

You can't: Only an eye care provider such as an ophthalmologist or optometrist can make that distinction.