In addition to staying hydrated on a daily basis, it's important to know the signs and symptoms of heat-related illness.
Symptoms: Someone with heat exhaustion may experience heavy sweating, weakness, cool, pale, clammy skin; fast, weak pulse, muscle cramps, dizziness, nausea or vomiting, headache, and fainting.
If someone is experiencing these symptoms, move person to a cooler environment, preferably a well air conditioned room. Loosen clothing. Apply cool, wet cloths or have person sit in a cool bath. Offer sips of water. If they begin to vomit or symptoms worsen or last more than 1 hour, seek medical attention. Heat Exhaustion can lead to Heat Stroke so it is important to act fast and recognize the signs.
Symptoms: Someone experiencing a heat stroke could have the following symptoms: Throbbing headache, confusion, nausea, dizziness, body temperature above 103°F, hot, red, dry or damp skin, rapid and strong pulse, fainting, loss of consciousness.
If this happens, it is imperative to call 911 or get the victim to the hospital immediately. Heat stroke is a severe medical emergency. Delay can be fatal. Move the victim to a cooler, preferably air-conditioned, environment. Reduce body temperature with cool cloths or bath. Use fan if heat index temperatures are below the high 90s. A fan can make you hotter at higher temperatures. Do NOT give fluids.
Source and more information: https://www.weather.gov/safety/heat-illness