Verifying codes
Video Tutorial: https://youtu.be/dKMYj3o_c2M
You should be verifying your codes as much as that is possible before selling them. Here's a tutorial on how to do that for different studios and providers. This is not 100%. It doesn't work for everything, but should for most. Use a Private Browser/Incognito to verify all your codes.
Movies Anywhere
You can only verify studios that are MA on here as far as quality goes. So WB, Universal and Sony. WB will not always be accurate on quality here if you want to redeem on FAH/Vudu. You need to know what disc it came from to be 100% sure. But it will redeem the quality it shows on MA.
Go to https://moviesanywhere.com/redeem put the code in and if it's good it will ask you to sign in and show the quality. The one good point about MA is that it usually will not allow you to redeem the same quality code. So if you own SD and the code is SD it won't usually redeem it. It will just tell you you already own it. This might be a good option if you don't know the quality of the code and cannot verify it.