Ok so the motor in my 2010 f150 4 door lariat premium is blowed up but I've found a guy with another 2010 f150 xlt with the 5.4 for sale but he doesnt have the key so how can I be sure the motor is good with out actually cranking it????

 

You can't. All you can verify that it isn't seized. This deal sounds strange to me and I would look elsewhere...."Caveat Emptor".

 

For sale for how much? Over $500, its not really worth it to run the tests....
What do you mean your engine is blown up?
If you pressure test the coolant system, and run a cylinder leak down test and cranking compression test on the engine, and it passes all of those, it's unlikely that you won't be able to mix the 2 blocks together to get a good running engine..... but you're looking at a solid day of testing to get that answer, and if you don't already have the tools, a couple of hundred dollars worth of tools at Harbor Freight prices.... More if you want quality.