Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
Seek emergency help if you or someone else has:
Move a person with possible heat illness to a cooler place while emergency help is arranged.
Do not force drinks if they are confused, unconscious, or unable to swallow safely.
A baby who is difficult to wake or appears seriously unwell needs emergency help.
Get urgent medical advice if fluids cannot be kept down, vomiting or diarrhoea is continuing, or signs of dehydration are getting worse despite drinking.
Blood in vomit or stool, severe abdominal pain, or black, tar-like stool also needs prompt assessment.
Babies and young children should be assessed promptly if they have markedly fewer wet nappies, no tears, unusual drowsiness, a very dry mouth, or difficulty drinking.
And for anyone, someone who:
Arrange routine follow-up if dehydration keeps returning, you often feel unusually thirsty, you urinate much more than usual, or a medicine may be contributing.
Routine advice is also appropriate if you need help making a safe hydration plan for a long-term condition.
Every sign above is quoted from the sofidex article on Dehydration. If you are not sure, err upward — it is always reasonable to be checked.