Would like to make bootable mSD cards for my Android devices.
How do I Check if Android Bootloader Allows SD Card Booting?
Given that we first understand that there is not a such a thing as the
and that all device producers actually can freely decide how to get the system to boot the Android OS's linux kernel it is hard to suggest a way that will allow you to test that for all devices.
Instead the way to go would be to use the information you have about the device (i.e. its name, model number etc etc.) and search the web.
Often the Bootloader is guessable by the used CPU Manufactorer. For example the Allwinner CPUs are mostly using U-Boot as the bootloader and to my understanding also have hardware dongled support to load from SD card (http://linux-sunxi.org/U-Boot)
Observe that I was hinting on CPU Manufactorer, i.e. to make understand that the bootloader pecularities of Android devices (that most often run on an ARM architecture) depends highly on them. That is that with Qualcom, Texas Instruments, Samsung etc... even though ending up with a comparable CPU (in terms of instruction set i.e. ARM v7) many things i.e. like the bootloader type/code can be quite different.
To answer you question I hence suggest you and all other people that face the same question to become aware that the Bootloader is the CPU and Device producer thing, not a standardized Android thing.
In other words, the Bootloader is not like "windows" (on a PC), not even like the NTLDR (on a PC), but rather it is like the BIOS/EFI on a PC. And as with the BIOS/EFI it is not the OS-guys (i.e that would be Microsoft, Redhead, GNU on a PC) but the Mainboard-Hardware-guys that provide for the BIOS.