what are signs of dehydration in toddlers? please answer!!!

My son is sick and won't eat or drink. He had wet diapers during the day but now it's dry. He's sweating as he sleeps. He's tossing a lot. Should I watch him tonight? How do I know if he's dehydrated if he's sleeping? 
He absolutely will not eat or drink ANYTHING. No popsivles, no crackers, no juice or water. Nothing!!! Ugh im at a loss