How to check what method is used by dispatch?

General Usage
2

Hopefully there are better ways to do this, but one option is to use which and check the (internal, private, unreliable, use at your own risk) field sig:

julia> using Test, InteractiveUtils

julia> abstract type A end

julia> struct B<:A end

julia> struct C<:A end

julia> should_return_one(::A) = 1
should_return_one (generic function with 2 methods)

julia> should_return_one(x::B) = 2
should_return_one (generic function with 2 methods)

julia> which(should_return_one, (B,)).sig
Tuple{typeof(should_return_one), B}

julia> which(should_return_one, (C,)).sig
Tuple{typeof(should_return_one), A}