Do a background search on the company it came from. If the email touts to be from an official company but the address is from a free email server domain like Hotmail or Gmail, find the company’s website through a search engine. Contact the company directly to determine if the email address really exists.
Use an email verification program like one at Broadband Help (see Resources). Type the email address into the service's system and see whether it determines that the email address actually exists. Test out the system by using known email addresses like your own and those of your acquaintances. Then test a random one of your own making. Be sure to type the address in rather than simply copying and pasting.
Compare the email address with the list of known fake email addresses at Web Angel (see Resources). This service was created in the mid-2000s, but the information is still valid.
Look hard at the email address. If an email address looks like someone smashed a fist into the keyboard to create it, unless by some strange chance you actually know that someone, it is fake.
Create a fake email address yourself and send an email to the address you're seeking to verify. If it goes through and you don't get a return email saying the address doesn't exist, then the email address is confirmed to exist.