How can i tell if im dehydrated i have a terrible head ache, can barely eat/drink i've been dehydrated b4 but it was much worse, could this b the start?

Only way to know for: Sure is to do blood tests. But if you have had little oral intake and have been losing fluid through diarrhea, perspiration, or bleeding, you probably are indeed behind on fluids. The quickest way to replenish, if you can't take oral fluids, is intravenous fluids. Go to emergency dept now, so doctor there can tell for sure and treat immediately.