When a person experience heat exhausting they're usually sweating a lot and they're experiencing a fever of 104 F or higher and the person will usually have clammy and cool skin, nausea, fainting, muscle aches, slow heart beat, dizziness and weakness.
A person with Heat Stroke will have dry skin since they'll stop sweating and they can also experience the above symptoms but no sweating and they may also have a throbbing headache as well.
Either way you need to seek medical attention for either heat exhaustion and heat stroke as both can be life threatening.
Heat Stroke is always serious although both of them can be.
Call 911 or seek medical attention right away if you think you or someone is having heat stroke or experiencing heat exhaustion.