How to check if a device can be made root-able
Rooting is not something endorsed by vendors (otherwise they would provide official instructions for doing this, or even ship the phones already rooted). Sony provides instructions to turn your device into a developer tool by unlocking the boot loader and changing the OS to a custom build. Motorola once offered the same. Also Nexus devices are friendly to rooting. 

But in general there's no simple way to answer your question. Every user is on his own with this problem. Yes, I know this is disappointing.