look at the MICR (Magnetic Ink Character Recognition) numbers that are located at the bottom of every cashier's check. They should be clearly printed, uniform in size and print, and not be crooked or smeared if the check is for real.
Real cashier's checks have a microprint signature line and there is no text of any kind underneath that line.
A legitimate cashier's check has a microprint border around its four edges. It also has a watermark printed on the back. But, a scammer can easily print that on his or her homemade checks too.