Heat Stress is a very easy issue to spot as the signs are unique. Heat stress is caused by temperatures being too high in the grow room for too long a period of time. What will eventually happen is the leaves will "Taco" upwards or fold up in half long ways, like a canoe. After a while of high heat, the leaves will begin to curl inwards from the tips and will burn. The plant will also slow down in growth when stressed by heat, and can even come to a complete stop.
How to Cure:
When it comes to fixing Heat Stress, the answer is pretty simple. Turn down the heat in your grow space anyway you can. This can include opening vents, adding fans, adding a mini AC unit, and more. If you are growing in a greenhouse, all you need to do is open all the vents and doors until you have lowered the temp to 75 degrees F. Having a grow light too close to the tops of your plants can also cause Heat Stress, so put your hand underneath to see if that's the problem. You may have to raise them up a bit.
Cooling Tower Fan