How would i know if i have a hairline fracture in my foot?

A foot x-ray: Foot x-rays will diagnose most obvious fractures of the foot. There are less obvious forms of injury, such as a stress fracture that might require specialized testing (such as a bone scan or mri). These can detect bone injury where normal x-rays cannot.

X-ray: Only sure way is to take an x-ray, however in some instances x-ray do not demostrate the fx but clinically you feel there is a fx(stress fx) a bone scan with help in making that diagnosis.