A forklift should be checked well and given a test run prior to purchasing it. This applies specifically to trucks being purchased on-line. It is really important to detect potential issues with trucks bought from companies on the Web.
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 particularly essential with trucks which have IC engines.
Testing the performance of the truck in challenging situations must be included in the test run. A skilled operator could drive the truck in forward and reverse, up the ramp.
You can inspect an IC engine truck yourself. On a clear part of floor, run the truck around for a few minutes while a trained operator tests the hydraulic functions. Afterward, examine the ground underneath for signs of fluid leakage. Even seemingly insignificant leaking can indicate serious issues which would be really expensive to repair later on.
A skilled individual should inspect the chains and masts. Check with and without a load that the mast works well. Watch for 'binding' or sticking of the mast. A warning indication is play in the mast channels or between the mast and fork carriage. One more is creeping down or creeping forward of the mast when loaded. These could be signs of issues with seals, ram or valve.
The mast may have been changed at some time if the truck is not new. Make sure that the mast and any fitted attachments follow the rating plate on a truck. Check that the sideshift is not sticking when loaded.
When checking the forks, look closely at the heel thickness for damage or wear. The truck's manufacturer specifies minimum thickness. A loose fit could mean the carriage or hanger points are worn. Look for cracks on the arms at mountings and heels.