Much like everything involving Android forensics, there is no one guaranteed method to determine whether a bootloader is locked, as it varies by manufacturer. To boot into the bootloader, use the adb command:

adb reboot bootloader

The device should boot to a screen that shows information regarding the bootloader. Frequently, this screen will display the bootloader status, as seen in the following screenshot.

The following is a generic, stock fastboot menu from a Nexus 5. Note that the Lock State indicates that the bootloader is unlocked:

The following is a standard Samsung Odin mode screen; Odin is the Samsung proprietary equivalent to fastboot: