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

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.