I change my Yukon engine but I am sure the auto shop didn't change it and charge me 2800 dollars how can I find if he changed engine or not

My car has the twist switch on the dash to engage 2wd, Auto, and 4wd. The vehicle engages in 2wd and the Auto slip with no problems, but it will not engage in 4wd. What is causing my 4wd not to engage?