Is there any way to tell if the CPU is fried beforehand? I don't think it is because I used the same cooling setup as the old CPU (ryzen 5 2600) when I popped it back in last night, and it worked fine. I have a hunch that the motherboard just doesn't want to handle the 3700x. Also is it safe to leave the 3700x in the plastic it came in while i'm waiting for the mobo? It's not going to move at all, it's just sitting in my desk.
I know of no way to test a CPU out of a system or without a special purpose test rig of some sort.
CPU's don't operate in full performance modes until everything is initialized and the OS starts to load up. I don't even think all 8 cores are enabled in BIOS as they're not needed. Although, CPU power saving features are also not enabled, that's why they can get pretty warm just in BIOS on only one core. But the CPU may never have gotten hot enough and even if it did protections might just shut it down before it got to the point of damaging it. So, you may be correct and the problem really is that your system simply can't handle a 3700X....either that or properly clamping on the heatsink will cool it correctly and allow it to work. But B350's never were intended to run 3rd gen and it's a bit dodgy trying to run one on them even when it works right.
Lastly; 3rd gen CPU's are a far cry different from any of the 1st or 2nd gen. They're designed to run very close to silicon limits by design. That makes them far more sensitive to cooling problems. Just because it works with your 2600 is no reason to assume it will with a 3700X.
Assuming this is what you mean, it's perfectly safe sitting in the clam shell package it came in. It's designed to protect the delicate pins as well as against electro-static discharges.