A forklift should be inspected correctly and given a test run prior to purchasing it. This applies specifically to trucks being bought on line. It is important to detect potential issues with trucks acquired from businesses on-line.
Starting the engine from cold is a good idea. Note whether there is too much smoke and the ease with which the engine turns over. This is especially crucial with IC engine trucks.
The test run should include an inspection of the truck's performance in tough situations. A driver who is qualified can operate the truck in reverse and forward, up the ramp.
You could inspect an IC engine truck yourself. On a clear area of floor, run the truck around for several minutes while a qualified operator tests the hydraulic functions. Afterward, look at the ground beneath for signs of fluid leakage. Even seemingly insignificant leaking could indicate serious issues that would be very costly to repair later on.
A trained person must inspect the chains and masts. Check with and without a load that the mast works smoothly. Watch for sticking or 'binding' of the mast. A warning sign is play in the mast channels or between the fork carriage and mast. Another is creeping forward or creeping down of the mast when loaded. These can be signs of issues with seals, ram or valve.
If the truck is used, the mast might have been changed at some time. Make sure that the mast and any fitted attachments adhere to the rating plate on a truck. Make sure that the sideshift is not sticking when loaded.
When inspecting the forks, look closely at the heel thickness for damage or wear. The truck's manufacturer specifies minimum thickness. A loose fit can mean the carriage or hanger points are worn. Look for cracks on the arms at heels and mountings.