How Do You Actually Know It's Trimmed?

[Buythatcard]

I am sure this has been discussed in one form or another but I am just trying to clear this up in my own mind.

How do grading companies or the everyday collector actually determine that an early vintage card is trimmed? Do they measure it? This is what I usually do but I have my doubts sometimes.

What guarantees are there that the machines that cut these cards in the old days were precise. If the machine is set to cut a card 2 5/8" wide but actually cuts it 2 1/2", will this card be graded 100 years down the road as been trimmed?

I just think that some of these early vintage cards that don't measure up to the average size of that card are not really trimmed.

Would love to hear what you guys have to say about this.