The best way to find out is to ask the developer or to try them.
I understand that scenery
developed in strict accordance with the FS2004 SDK will probably be OK. Some that went beyond the SDK are known not to work properly. Also, the FS2004 SDK said only XML should be used and warned against using BGL opcodes and the BGLC compiler from previous versions because these may not be supported in later versions of FS. I understand that FSX doesn't support them.