A forklift should be checked well and given a test run prior to buying it. This applies specifically to trucks being acquired online. It is important to find potential problems with trucks purchased from companies online.
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 must include a check of the truck's performance in tough situations. A trained driver can operate the truck in forward and reverse, up the ramp.
You can check an IC engine truck yourself. On a clear area of floor, run the truck around for a few minutes while a trained operator tests the hydraulic functions. After that, look at the ground underneath for indications of fluid leakage. Even seemingly insignificant leaking can result in serious issues that will be really pricey to fix later on.
A skilled individual must check the chains and masts. Check with and without a load that the mast works well. Watch for sticking or 'binding' of the mast. A warning indication is play in the mast channels or between the fork carriage and mast. One more is creeping forward or creeping down 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 used. Make certain that the mast and any fitted attachments follow the rating plate on a truck. Make certain that the sideshift is not sticking when loaded.
When inspecting the forks, look closely at the heel thickness for wear or damage. The manufacturer of the truck specifies minimum thickness. A loose fit might mean the hanger points or carriage are worn. Look for cracks on the arms at heels and mountings.