A few things to look at with the way the lift was done.
Did they cut or modify the factory mounting points on the frame or steering? Any cutting of the frame will void any warranty, and most likely allow the manufacture a way out for any claims or injuries.
Did they lift via blocks between the springs and axles? Mostly found on older leaf spring vehicles.
Did they replace the factory springs and shocks, etc. with after market? May or may not have the same load ratings as OEM parts.
Did they just lift the body with spacer blocks? Ok for 1 or 2 inches of lift in most cases.
Was the modifications done by a factory approved vehicle modifier or a local garage? You some times see units at new car dealers that are done with the blessing of the vehicle manufacture, and will still carry a new vehicle warranty.
Will the lift kit supplier provide the load capacities in a letter? This way you can make sure you have the needed capacities you need.
By the way what make of vehicle was it?