Mostly, people check the load rating using the given model number from the officical site of manufacturer. But you can use anything for a light machine. I mean a 15mm lm guide can mill steel very well if used with proper base of steel or iron casting. You just make sure that you use lm guides and not round rods when making a machine for metal working. Round shafts (bushings) don't have enough load ratings. I have checked that a 15mm lm guide can take a load what a 3in supported road cannot take.
what you want to use it for btw?