One method is to submerge the card under water for an extended period. If the card starts to separate, it is real. If it does not, then it is fake.
Another method is to Light the corner on fire. If you see a green flame, the card is real. A blue flame signifies a fake.
Haha. In all seriousness, I do exactly what you are doing - Compare commons with marquee cards under a loupe. I check for print dots, texture, thickness, etc.
This is going to sound weird, but the card must also pass my smell test - a literal, not proverbial, smell test one. Old cardboard has a distinct smell to it, at least it does to me.