Testing return types of functions

General Usage
2

If you mean testing as in unit testing, Test offers the @inferred macro. If you mean testing as in checking what the return type will be before running g, you can also do this, but not through any exported methods (so beware).
You can use Base._return_type and Base.return_types

julia> Base.return_types(+, (Int, Float64))
1-element Array{Any,1}:
 Float64

julia> Base._return_type(+, (Int, Float64))
Float64

The return type is only as stable as your functions and inputs though:

julia> Base.return_types(+, (Complex, Int))
2-element Array{Any,1}:
 Complex{Int64}      
 Complex{_A} where _A
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