Most will have had a small sticker on them at one point that said "isolated power required". On the other hand the adhesive on most of those stickers baked off years ago and they have dropped off.
So, if it is a WG46xx, WG49xx, G07, or Sanyo 20EZ it definitely needs an isolation transformer.
It'll be pretty obvious by looking at it - monitors that require isolation transformers simply rectify the line and filter it. Monitors that don't require isolation transformers have switching supplies built in, so it'll have the switching transformer, chopper transistors, etc.