Isolation monitors are needed on just about all of the the older models of monitors. WG, Electrohome, Pentranic and Sanyo for sure. If it doesn't come with a three pronged "plug in the wall" power cord, it probably needs an isolation transformer.
Although there was probably a warning sticker when it left the factory there is no guarantee that it has stayed on over the years. If you have a specific monitor in mind, just ask. Somebody here probably has one and can give you a definite thumbes up thumbs down.
ken