i just got a quick question thats prolly been answered before

[blackdragon709]

what u guys do to determine when someone would be getting exhuasted?

 

[Crothian]

They make a con check, the rules are in the DMG.

 

[Nareau]

From the SRD:

Exhausted
Characters who are exhausted move at half normal speed and suffer an effective penalty of -6 to Strength and Dexterity. A fatigued character becomes exhausted by doing something else that would normally cause fatigue. After 1 hour of complete rest, exhausted characters become fatigued.
Fatigued
Characters who are fatigued cannot run or charge and suffer an effective penalty of -2 to Strength and Dexterity. A fatigued character becomes exhausted by doing something else that would normally cause fatigue. After 8 hours of complete rest, fatigued characters are no longer fatigued.

Also, most things that cause fatigue can be found under the "Environmental Conditions" section of the SRD:
http://www.opengamingfoundation.org/srd/srdenvironment.rtf
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They're generally a Con check or a Fort save, DC 10 (or 15) +1 for each previous check.