Without its unique phone-specific WiMAX RSA encryption keys, the "4G" in HTC EVO 4G wouldn't work. The keys are required for 4G to function properly and their presence on your EVO is one of the first things to check for if you're suddenly experiencing connectivity issues.
The keys can sometimes break or get lost when you flash something, so it's always good to know that they're there, especially since I believe the only way to get a new set of keys is to get a new EVO.
There are several different ways to check for the RSA keys on your EVO, but I think the quickest and easiest one to use if your phone is rooted is joeykrim's Wimax Keys Checker.
It's free in the Market and as long as you have BusyBox installed (and you probably already do), it takes just seconds for the app to determine whether your EVO's got the keys to the WiMAX house.
All you have to do is press the "Check for RSA keys" button and cross your fingers that the result will be one of congratulations.
You can have Wimax Keys Checker double-check whether your EVO is rooted and has BusyBox installed too (tap the appropriate buttons), but it isn't necessary if you only want to know the status of the keys.