So I made an automatic updater that will update rust and umod whenever https://umod.org/games/rust/latest.json has a different version string.

But today it triggered and rust did not update even though steamcmd said it downloaded something. 

Is there any way to check the version of rust installed to make sure steamcmd got an updated version of rust?

That .json feeds are cached, but should be up-to-date at this moment. For Rust, you'd have to parse that from the game's DLLs.

How long is it cached for? What data source should I use instead then?

And is it possible to match the umod version with the version of rust it is supposed to work with?

And which DLL do I parse the version from?

Assembly-CSharp.dll is where the Rust version is stored I believe. The other questions Calytic would have to answer.