This involves some effort, but it may well be worth it in cases where repeat offenders repeatedly barrage you with unwanted e-mail forwards. Note whether the text was actually written by the person who sent it to you. If not, be skeptical. This usually means the opposite of what it says. If the message seems geared more to persuade than to inform, be suspicious.
Hoaxers are out to push emotional buttons.
If the message purports to give you extremely important information that you've never heard of before or seen elsewhere in legitimate sources, be suspicious. Read carefully and think critically about what the message says, looking for logical inconsistencies, violations of common sense and obviously false claims. Check for references to outside sources. Hoaxes will not typically name any, nor link to websites with corroborating information. Be especially wary of health-related rumors.