It can be pretty difficult to tell when you go from being hot to being impaired.

Things to look out for:
- Concentrated urine (or not needing to go). Drink enough to keep it pale and flowing! (Nb. If you are sweating heaps, you need electrolyte replacement as well as water replacement).
- Headache
- Cramps
- Feeling vague / disoriented / off balance

Unfortunately it kind of creeps up on you and it's hard to tell the difference between being tired and being overheated and dehydrated. You need to be proactive, eg. hydration and resting when the temps start getting ridiculous.

With the sun, just cover up. You don't notice sunburn until it has happened. Same with melanoma.

Sunscreen protects against sunburn and against squamous cell and basal cell carcinomas, but it is not clear that it protects against melanoma. Shade/clothes, sunglasses etc. do, however.