Hey Alex, hey all,
Yes! Exactly.
Simply stick your milling bit in the four corners of a rectangle and then check if it really is a rectangle (or a parallelogram instead). In a rectangle the two diagonals are equal.
In the discussion, we have to parties.
The ones say, ensure perpendicularity by comparing it with a reference object.
The disadvantage is that a reference object, already of the size needed with Onefinity Woodworker Machine (32″), i.e. a precision square of sufficient length and at the same time sufficient accuracy, is quite expensive, a three digits sum if you want that accuracy and size.
The others, including me, say, ensure perpendicularity through geometry.
With this, you don’t need expensive measurement tools or such things. You don’t even need a ruler. Take a stick and saw it off so that it fits diagonally between the opposite ends of the object you want to “square”, and then turn it between the other two opposite ends, and if it fits equally tightly, then the diagonals are the same and the angle is right (= the machine is “square”). That’s all.
You can make it more comfortable by making your stick out of two parallel sticks that you can move against each other and fix them with Bar Gauge Heads. It’s called Bar gauge then.