The first thing you should do is make sure there is no physical damage to the phone. This may sound pretty straightforward, but it can be deceptive. The number one killer of cell phones is liquid damage. When you hear that you probably think that means a phone was just dropped in the toilet, but believe it or not, something as simple as sweat can damage a phone when it is in your pocket. To make sure your phone does not have liquid damage, take out the battery and look for the indicator, often several of them in the phone that will tell you if the phone you are looking at has such damage (see my full article on this). If the indicator is any other color than white, stay away.

Other physical damage may not be readily apparent. Check the screen to see if there are any lines or weird colors in the lcd display, or if it looks oily or like watercolor. This may indicate the lcd is cracked inside. Check the casing to see if there are even hairline cracks, which can grow and become big problems in the future. Often these are hidden by cases or covers, so remove those to check before you buy. Also, check the battery and contacts to make sure they don't look burnt or discolored which might be an indication of excessive heat. Basically, just look for anything out of the ordinary.

Don't trust the person who is selling you the phone. I know that may sound harsh, but he has every reason to lie to you to get rid of his piece of junk phone, and you have to be on the lookout to make sure you don't get a lemon. If someone says they have never dropped the phone, never taken it out of it's case, only took it to church on Sunday in a protective bubble filled with packing peanuts, etc., I would be doubly suspicious. Does the phone look like that? If it is beat to death and someone is telling you they treat it like their child, you might want to call child protective services. If on the other hand someone is honest and says "Yeah, I dropped it a couple of times, and my dog used it as a chew toy but it never gave me a problem," that is not necessarily a bad thing as long as the price reflects the condition and you can live with it.

The main things you want to check are the phone's functions. Is it able to do the things you want it to do? If you have a phone with a sim card, put YOUR sim card in it to test. Does it make calls and sound fine? Does the speakerphone work? How does the camera work? Is it able to send and receive text messages? You can send a text or picture to yourself to find out. If it is a smartphone, see how it does with email and synchronization. See how the browser works and if it is fast, or bogged down by a lot of unnecessary junk the previous owner downloaded. If the seller tells you he doesn't have time for you to test all this, find another seller.

Remember, you are probably going to be using this phone for a year or two, so just a little time to test can go a long way to making sure it will function appropriately. A few minutes of testing could save hours of headaches later on.