I got an email from American Express

which said that I am eligible for a credit limit increase if I update my income. My question is, how would AmEx know if I am telling the truth.

For context, the only account I have with them is a Blue Cash Preferred credit card that I have had for the past 2+ years. I have always paid the monthly statement balance in full. I last updated my income with them when I applied for the card 2+ years ago.