What variable type is this?

I need to rotate my object and then save the rotation in a variable, so I tried

RotationObject1 = Object1.transform.Rotate(Time.deltaTime, 1, 0);

But I need to declare the variable and I have no idea what type it is? (Transform, float, int, boolean or similar)

Transform.Rotate does not return a variable.
What it does is change the rotation of the transform, in your case it will chagne the rotation of Object1.

So Object1.transform will contain the new value after you run that command.