Q: Macbook Pro after service/repair - what to check?

Hi, my two month old Macbook pro crashed and needed a repair under warranty.  The motherboard and screen had to be replaced.

When it came back I wasn't able to log in (my password wouldn't work) but I looked for solutions and was able to use recovery mode to change the password and that now works.  I have lost confidence in this Mac and equally in the repair if they can't even leave it with my password working plus they didn't explain what the original problem was. 

My question is what tests should I run to check that everything is working as it should?