How to determine what the repair people did to fix my MacBook? Recently, my MacBook stopped powering up. When pressing the power button, nothing at all happened. The usual green-yellow light on the power supply appeared normal, but no lights or sounds came from the MacBook itself. Only very rarely, after trying many times throughout the week, would it power on after pressing "power on". Once on, it never powered it off without me instructing it to shutdown.

I took the MacBook to a repair store. They said I should leave it for a few days. When I returned, it worked, albeit they lost a few of the screws, didn't completely bolt the parts firmly together, and got it really dirty. They refused to give me any details about what they did to fix it, but showed me that it works fine now. I don't know if they made significant changes, such as replaced internal parts, or just wiggled some wire and got it working again.

Are there any steps I can take to investigate the computer to identify how the repair store fixed?
I work at an Apple Authorized Service Provider, and the fact that they won't tell you what they did means it was probably just an SMC reset that took two seconds. From Apple Support:


  The System Management Controller (SMC) is responsible for many
  low-level functions on Intel-based Macs. These functions include:
  
  
  Responding to presses of the power button
  


I'm guessing they didn't tell you this because all it takes to reset the SMC is holding down 3 keys and the power button at the same time while plugged in, and you'd possibly argue about having to pay them. It seems pretty shady of them, because we have machines come in needing a simple SMC reset all the time, and most of the time we waive labor charges, but for them to not even tell you that that's what they did doesn't make a lot of sense.