Given how a copy of a class variable can change the values of the properties in the original, it seems pretty important to know whether the object you're dealing with is a struct or a class.

If you've written the code yourself or have access to the source, that might not be a problem, although it might be a lot of work. But if you have a compiled library (?) and little documentation, is there any way for a program to tell if something is a struct or a class?

Do you want to know at compile time or at run time?

At compile time, you can do this:

thing1 === thing2

and you will get a build error if thing1 and thing2 are not classes or other reference types.

I can't honestly see why you would need that info at run time.