Nobody answer? Nevermind, I will use my experiences and tell you. So if anyone would like to add further details or do some corrections, please do so 

Firstly, check the outlook for both Body and Lens, if you're happy with it, then proceed to the next stepsssss. 

For the Body:
1. check the Shutter Count, the lower the SC, the better it is, just like a car's milage. Google has a clearer explanation.
2. if the Body has motor and top LCD, make sure they both work well, it's sux if the motor doesn't work, it's sux if the top LCD can't light up.
3. test the Body with various memory cards, you don't want to have error while shooting, don't you? (sometimes it's the memory card that give trouble)
4. test Continuous shoot, will the shutter jam? 
5. basically you'll have to test everything, every single functions and settings of the body, like ISO, pop-up flash, the condition of hot shoe, the buttons etc etc

As for the lens:
1. it's important that the rubber is still firm.
2. make sure it works on more than ONE Bodies.
3. play with aperture, the zoom if it has, play with it like a toy.
4. if the lens got dust and fungus inside, you can send it for cleaning.
5. if the contact points are dirty, gently use eraser to clean it.
6. if the lens is AF-S, make sure the motor inside is silent while auto focusing, no loud squeaky sound.

what else?   

Remember, these are based on my personal experiences, correct me if I'm wrong