How do I know what extensions will be incompatible ?

You can check the legacy software section in the System Information/Software section (hold down the Option key and select System Information in the  menu).

Have a look at > https://support.apple.com/en-us/HT210999

and > https://support.malwarebytes.com/hc/en-us/articles/360045251654-Legacy-System-Extension-warning-on-Malwarebytes-for-Mac-devices

Concerning Apple’s announcement regarding Legacy System Extensions, there is nothing we have to do or be worried about at this time. What Apple is telling us is that when the next new operating system comes out (Big Sur), it will no longer allow those Legacy System Extensions which are part of other vendor’s apps. That means that any software vendors who have failed to update their offerings will be incompatible with the new OS. Those vendors have all been told what changes they will need to make to be compatible and I assume they are doing so. The bottom line is this. Before installing the next OS we need to visit our vendor web sites to verify that they have updated their offerings. If an important software has not been upgraded, then it might be a good idea to wait before installing Big Sur. Some people ask why Apple does not identify what programs use these extensions. This is because sometimes more than one vendor may use the same extension. In the meantime, it is nothing we need to worry about.

I take that as you don't have any legacy extension. I don't have anything listed either and everything works in the beta system.