Determining whether your bootloader is locked or not
Originally Posted by TX_Jax
How do you confirm an unlocked bootloader?
You have to get the phone into Odin mode, as if though you were going to flash something via Odin.
First, take the back off, as you will most likely have to pull the battery to reset the phone at the end of this.
With the phone powered off, hold Volume Down, the Home button (or whatever it's called) at the bottom, and Power at the same time, until the phone turns on, and you see a Warning screen. On that warning screen, press Volume Up to Continue.
The phone will now be in Odin mode, and you'll see an Android with the text Downloading..., Do not turn off target!!
Look at the small text in the upper left corner of the screen.
If you see mention of Knox Warranty, your bootloader is locked, and currently cannot be unlocked.
You may safely pull your battery despite the warning at this point to exit Odin mode (I don't know of any other way to exit Odin mode, unless I'm simply not holding down power long enough to do it). Pulling the battery at this point would only be an issue if you were actually using Odin to flash something and you pulled the battery in the middle of the flash.
Note: Some people may be tempted to try and use adb to achieve this instead of powering down the phone, but this will not work; "adb reboot download" will put the phone in download mode, but you will not see the text in the upper left if you try this method. The only other sane option that I can think of to try would be "adb reboot odin", and this does not have any effect other than rebooting the phone normally.