[Sammy-ROCK!]

How can I hook a command and detect if a plugin has blocked it?

[Kigen]

Like knowing what commands have been returned "Plugin_Handled"?

[Sammy-ROCK!]

Only knowing if any plugin returned Plugin_Handled or Stop.

[Kigen]

Only possible way is to see if they pass through OnClientCommand().

[BAILOPAN]

File a bug report. This is a general problem in SourceMod where you can't see the event passthru status, like you can in Metamod:Source. It deserves a solution.