First off, never beat yourself up about trimmed cards. We have all bought them and will continue to do so as long as we collect. Unfortunately, it is part of the hobby. Many times collectors are unaware cards are trimmed. I agree with many of the comments that sizes vary on the cards so measuring isn't always the answer. For me, the easiest way to learn was to take a card from almost every year and trim just enough off of one side but leave it attached. I attached a photo. You can now look basically side by side with a true raw edge and a trimmed edge. It is amazing when you compare this way how different they actually are. Look under a loop and it is night and day. It takes a while, but pretty soon, you can pick them out pretty easily. Just my 2 cents..
