Often, when one is suffering heat exhaustion or the beginnings of heat stroke, the symptoms will show up first - confusion, disorientation - not knowing what is up or down, right or left, red patches on skin, dizziness.
Thus, should one encounters such a person, immediately evacuate him/her to a shady area & lie down. His/her body temp would be rapidly rising along with his/her heart rates. Calm him/her down & tell him/her to calm down, to slow down the heart rates. Remove any thick outer clothing that he/she may be wearing - such as jackets, ties, etc, to allow better air circulation & heat transfer to the surrounding area.
This must be done before calling for medical assistance, as such assistance will take at least more than 10mins to arrive, & many things can happen to that person with rapidly rising temp & heart rates, which will burn up Human organs, within 10mins...
First aid for heat exhaustion or heatstroke victim. From Mayo Clinic, USA :-
Fan the person while misting with cool water. Place ice packs or cool, wet towels on the neck, armpits and groin. Cover the person with cool, damp sheets. If the person is conscious, offer chilled water, a sports drink containing electrolytes or another nonalcoholic beverage without caffeine.