A print guy in Denver held my business card under a lamp and said one word: "nope"
I handed him a card with light gray text on a white background, thinking it looked clean and modern. He flipped on a desk lamp, laid the card under it, and said "nope" without even looking up at me. Standing right there, I couldn't read a single line of my own phone number under that light. That was two years ago and now every color I pick gets the lamp test before it goes anywhere. What's the fastest trick you all use to catch low contrast before a client sees it?