If you don't mind, give us the details Raving Loony. I've said that it isn't that common for Harm Reduction workers to encounter hyponatremia (at least in our neck of the woods) so I would like to hear your full account.
LahLahGatecrasher; The usual visual signs of dehydration/rehydration such as fatigue, sweating, red face etc. are the things Harm Reduction workers look for when roving an event. If someone requires assistance from Ravesafe, the worker will ask the person (or friends) if they have a headache or are otherwise feeling unwell. If the HR worker is convinced the person hasn't been drinking too much water then they will encourage him/her to drink. It's all about "feel" really, as there's no way of really estimating just how dehydrated someone is (for outreach workers anyway). Signs such as dark coloured urine, dry skin and bags under the eyes aren't that reliable either as other things may affect or cause the conditions.
MDMA causes body temperature increases. Even small amounts will do this, but individuals may be more or less prone to these actions of the drug. Health factors such as cardiovascular disease; thyroid, hypothalamus (temp reg centre) or pituitary disorders, or just a sensitivity to any of the drug's physiological actions may result in a person experiencing a greater degree of dehydration and overheating (the 2 usually going hand in hand).
If aiding someone, it's important to consider a number of factors before suggesting the person drink a lot of water. If there is ever any doubt, HR workers will refer the person to ambulance or emergency personnel.
For a personal check, I think the best you can do is to recognise the things which add to dehydration. Alcohol, insufflating anything and many other drugs by themselves cause dehydration. Dancing and other strenuous activity, environment (appreciating temp increases as room fills) and even your level of anxiety prior to dropping can add to the Temp. effects of MDMA.
When under the influence of MDMA, level of water intake is usually recommended as:
~250-300mL of water/ hour if not physically active (chilling)
~500-600mL of water/ hour if physically active (dancing, sex etc)
These are approximate only and as said various factors may mean
a person may require more or less water during their roll.
Hope this is of some help