You're not going to NOT exercise because it's hot out. In fact, there are plenty of benefits from exercising in the heat, but there are a few signs to be aware of when working out when temps get warm.
The obvious is, are you THIRSTY? Thirst is a natural alert that you hydration levels are low.
Next, are you experiencing an increased heart rate above what would be normal for your workout? This is a sign of increased core temps.
Are you feeling lightheaded, dizzy or disoriented? Another dead give away that hydration levels are off.
Do you feel nauseous?
Lastly, are you not sweating? Believe it or not, this is not a good thing when it's hot and you're not sweating
If you notice any of these symptoms here's what you should do.
1) Slow down or stop exercising altogether. You're not saving the world so cutting a workout short when these symptoms arise is ok.
2) Hydrate yourself. Seems obvious but the best thing you can do is add water.
3) Apply ice or move into the shade. Anything to bring that core temp down as you re-hydrate.