You're usually taught to yell for help as you start CPR. There's not really a way to tell if a person has a had a heart attack without an EKG or labs. You can assess for cardiac arrest (that's the whole reason for CPR), and any time the heart is stopped, the best chance for the patient is immediate, high quality CPR.
The only exception to this rule is if you are in an area where no one will find you unless you call for help yourself. For example, if you are home alone with someone and you start CPR, you will probably need to stop out of exhaustion before someone will hear you cries for help. In this case, you need to call 911 first. If you're in a mall or something, then leave the emergency call to someone else and get started on chest compression.
The only exception to this rule is if you are in an area where no one will find you unless you call for help yourself. For example, if you are home alone with someone and you start CPR, you will probably need to stop out of exhaustion before someone will hear you cries for help. In this case, you need to call 911 first. If you're in a mall or something, then leave the emergency call to someone else and get started on chest compression.