When something is a constant, I am pretty sure I know what its value will be. When it is a variable, I often need to know more. Languages don't really make this distinction. Often a constant is declared in a different way than a variable, but when you see one used you often cannot tell which is which without a convention. Moreover, if a variable is set once during program initialization and never changed, I consider it to be a constant whether the programming language does nor not.
Constants are all upper case letters and have more than one letter.
Variables on the other hand are not all upper case letters unless they have just one letter. Variables do begin with an upper case letter. I find it valuable to distinguish functions that have no side effects from all other forms of subprograms. I want to know when I see a function invocation that there are no side effects. I name functions without side effects the exact same way I name variables.