I assume by that you mean that you are picking it up and want to know what things to check before handing over the cash and accepting the item as working.
You almost certainly cant open it up to check internal components.
Get the seller to show you it working, including steaming milk.
Check that there is no water leaking around the portafilter when they pull a shot.
check that there is a small dribble of water/coffee into the drip tray from the chrome tube on the left hand side (this is the three way valve operating) when they switch off the pump.
When they turn the steam switch on, listen for any hissing indicating leaks, check there are no drips coming from the steam arm until it is turned on.
Basically, as long as it heats up, pumps and doesnt leak, should be ok (may not be perfect, but these are pretty simple and robust machines).
Chances are they will not have a blind* filter. Buy one and some cleaner, then learn how to backflush - a quick search should give you the info.