How can i check if a client is compatible with a server version of WCS3?

We upload tested builds client+server here: https://flashphoner.com/download_wcs3/
If you follow the link and download build, you will see:

For example:

WCS3-Client-3.0.419-Server-3.0.520

Client build: 419
Server build: 520

If you unpack and open the archive, you will see the client hash:
WCS-client-3.0.419-db3249957093269354e77f7988c8fd6f138118b1

It’s commit Nov 13, 2013
https://github.com/flashphoner/flashphoner_client/commits/wsc

So, you have exact version of client source code db3249957093269354e77f7988c8fd6f138118b1 that should work with server build 520.

If you are developing your own application, you should follow these changes.
Note, you have full access to the client source code for all versions.

It’s possible situation when too old client is not compatible with a new server version.
In that case you should correct your client or downgrade server to previous compatible version.