For tooling purpose I would like to have a reliable check to see if a symbol is
instrumented for debugger or not.

I have seen that internal code in debug.el uses advice
'debug--implement-debug-on-entry' for instrumented symbols, but I am not sure if
it can be used as a stable check long-term. Suggested patch uses it, but if it
is considered internal and subject of change, please add a way to have a stable
predicate.