Is it possible to write a template to check for a function's?
The answer depends upon what you mean by the existence of a function. Checking for the existence of a function within the current visibility of a function call is the job of the compiler or the linker depending upon the particular language being used. This may be different than checking for the existence of a function. From a purely ontological point of view a function may exist in a source file or an object file not referenced by a particular program. In that case the compiler and/or linker will not even attempt to locate the function. There is no way for a program to check for the existence of a function on some foreign computer not connected to the programs computer through a network.