Not sure method_exists() is helpful, but it is sort of related ...
julia> function Foo(x::Float64,y::Int64) # lets call this f1
#some code
end
Foo (generic function with 1 method)
julia> function Foo(x::Int64,y::Float64) # lets call this f2
#some code
end
Foo (generic function with 2 methods)
julia> method_exists(Foo, (Float64, Int64))
true
julia> method_exists(Foo, (Int64, Float64))
true
julia> method_exists(Foo, (Float64, Float64))
false
