Juan,
I typically do not advise end users on how to perform mast inspections as they are a dangerous mechanism to work on, but I can give some general pointers. Never put your arms or legs through the upright where they can be severed if the mast drops unexpectedly, and never stand underneath or on top of the forks. Also, do not stand between the mast and the truck body as the levers could be activated unintentionally. If you raise the forks a foot or so, and tilt the mast all the way back, you can then crib the bottom of the mast with hardwood to check the condition of the mast support bushings and tilt cylinder pins by tilting the mast forward and observing relative movement of the tilt yokes against the pin supports, and the mast yokes against the axle tubes. With the forks raised to eye level, take a sightline down the fork shaft to check for bending, and take a sightline across the carriage bars to check for straightness. A gauge is required to measure the wear of the fork heels, which cannot exceed 10%. Use a prybar to move the inner rails side-to-side against the outer rail and if the play seems excessive, beware. (There is a more proper way to measure this which I would consider too dangerous for an unsupervised beginner.) Visually check all welds for cracks, all cylinders for leaks/wetness/visible drifting (accompanied usually by a periodic cracking/banging sound) and all hoses for cuts and slashes (hoses must be replaced if the stainless or nylon braiding is cut or damaged). A special gauge is also required to measure chain stretch (allowable 3%) but you can check chain pins for turning (all swage marks must be positioned perpendicular to the run of the chain and no joiner links may be used in the run of chain that travels over the pulleys) and excessive rust/lack of lubrication. Mast bearings typically fail at the bottom of the mast, but can be dangerous to check if the mast is not properly supported. If in doubt, pass it up.

I would suggest going with the popular brands rather than an off-brand/gray market truck, i.e. one that was not configured for the american market (they do show up in auctions or basement sales from time to time) take the model/serial number and ask the local dealer for a wheel cylinder or air filter and see if they recognise the model number. if they don't, run. The only exception I would make to this would be the Clark GCS/GPS/GCX/GPX series trucks as they did have problems at one point with the outer mast channels cracking at the mast support welds, and I have never enjoyed driving them or repairing them. the gear selector lever is enfuriatingly difficult to accurately shift into neutral, the floorboard weighs a ton and often develops deep rust scale that gets everywhere, filters are difficult to access, the cylinder head is expensive to service and sparkplugs often seize in them without antiseize, and don't get me started on the hub seals with the rotating inner member.... or the secondary chains of unequal length... and theyre just not comfortable...