Is it possible to have a heart attack and think it was your first panic attack instead? is there ways of telling after the fact?

Absolutely! EKG may: show evidence of heart muscle damage, most likely during, much less so later. Most heart muscle damage is not visible by EKG. Why? Small, endocardial muscle (inner wall)=most susceptible, overlying muscle voltage masks absence of voltage from previously healthy muscle, some muscle heals & scar tissue shrinks. The most sensitive late sign on EKG is elongation of the QRS duration. Healthy <<80 msec.