mvf.js

gistfile1.js

var M = function() { this.m = function() { return 42 } };

var inst = new M();

inst.f = function() { return 42 };

// How to tell that f is a function in a field and 
// m is a method in an *arbitrary* object?
// 
// example:

is_method(inst.f);
//=> false

is_method(inst.m);
//=> true

// Is it possible to write is_method?

In your example, the method assignments are functionally equivalent -- this in the constructor is going to be == your inst; the interpreter doesn't keep track of ("doesn't care") whether the value at a given field is assigned as part of the constructor's operation, or if it's assigned later. So the short answer is: "No, you can't write an is_method."

What you're looking for is probably something more like this:

var M = function() { this.v = 42; };
M.prototype.m = function() { return this.v; };

var inst = new M();

inst.f = function() { return this.v; };

inst.hasOwnProperty('f');
// true

inst.hasOwnProperty('m');
// false