A forklift should be inspected well and given a test run before purchasing it. This applies specifically to trucks being obtained on line. It is important to detect possible problems with trucks obtained from businesses online.
Starting the engine from cold is a great idea. Note whether there is excessive smoke and the ease with which the engine turns over. This is especially crucial with trucks which have IC engines.
The test run must include an inspection of the truck's performance in tough situations. A trained driver can maneuver the truck in reverse and forward, up the ramp.
You can check an IC engine truck yourself. On a clear part of floor, run the truck around for a few minutes while a skilled operator tests the hydraulic functions. Then, look at the ground beneath for signs of fluid leakage. Even seemingly insignificant leaking can lead to serious issues which will be pricey 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 indication is play in the mast channels or between the mast and fork carriage. Another is creeping down or creeping forward of the mast when loaded. These can be signs of problems with valve, seals or ram.
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. Check that the sideshift is not sticking when loaded.
When inspecting the forks, look closely at the heel thickness for damage or wear. The manufacturer of the truck specifies minimum thickness. A loose fit can mean the carriage or hanger points are worn. Watch for cracks on the arms at heels and mountings.