How do i make check if a variable changed

Like, can i make something like this?

if (A && ~A) {}

with ~A as being if A has changed.
I see this in Garry’s Mod E2 and its useful, but i don’t know how to use something like this in Unity JScript.

No, there is nothing like that in JavaScript or C# with Unity.

Perhaps the closest you can get is a setter function in C#, but if you explain why you actually want this, there is probably a simpler and more obvious explanation of how to achieve what you want (in particular, what is changing the variable?)

as Waz said you can identify one Boolean for instance MouseIsDown becomes True when GetMouseDown is called and becomes False when GetMouseUp is called. then you can refer to this variable to judge for you.