There is a great deal of ambiguity and misinformation in the previous answer(s) above. The root cause of this is ambiguity inherent in the words "flash" and "flashing": some people use a "flash tool" to "flash" .img or .bin files, others use a "custom recovery" to "flash" a .zip file, and still others use the bootloader to "flash" a .zip file.
Mobile Uncle MTK Tools (mentioned above) is software used with .img and .bin files, but there are others such as the so-called SP-Flash-Tools (SmartPhone Flash Tool). These tools are typically Windows, mac, and/or Linux programs. (and btw the correct word in this case is "download," not "flash").
When using a recovery to flash an update, the code that does the work sits in the recovery partition itself, and the user controls the process via some sort of GUI on the phone that the recovery displays.
When using the bootloader to update the phone, there is a special-purpose client called fastboot that lives on a computer which is connected to the phone via USB. Fastboot is also the name of the protocol that this client speaks over USB with the bootloader code on the phone.
With those three things understood, it's easy to see what's going on:
If you use a flash tool to download some .img or .bin file to the phone, then you don't use any code in the bootloader at all. So whatever it means for the bootloader to be "locked" or "unlocked" doesn't matter (since you're not using the bootloader)
If you are using fastboot and the bootloader, then unless the bootloader is UNLOCKED, you won't be able to install ANYTHING but a bona fide uncorrupted factory-approved update file. That's what it means for the bootloader to be locked -- if you tell such a bootloader "Fastboot flash mywacko-update.zip" or anything similar, you'll see an error message and failure. Because it's locked.
Now if you use a recovery to update the phone, you might think that we are back in the 1st case -- you're using the recovery and not the bootloader, so it shouldn't matter if the bootloader is locked or not. Although the logic is unassailable, the fact is that the stock recovery of any Android that ever lived contains code which will cause the update to fail unless the update is a stock factory-approved one. In other words, it's a fact that if the bootloader is locked, you won't be able to use the recovery to install something non-standard either.
But it's perfectly supported to take a flash tool and download a CUSTOM recovery which is not stock -- where one important feature of the custom recovery is that it just doesn't care about the locked/unlocked status of the bootloader.
If you do this, your custom recovery will "flash" anything you ask it to, .but you STILL won't be able to use fastboot and he bootloader to flash anything non-stock. Because the bootloader will still be locked.
Finally, to answer the question about how to verify if the bootloader is locked or not:
Boot into the bootloader (sometimes called "fastboot mode"), connect the phone via USB to a computer that is running fastboot, and type the following command:
fastboot getvar all
If you're lucky, among the many lines of output you get will be ones such as:
(bootloader) unlocked: no
(bootloader) secure: yes
Strictly speaking, I believe that locked/unlocked just means that: it can not be changed. The bootloader is "secure" if it will only install factory-signed images. That being said, I have never seen any combination but two: unlocked and not secure // or locked and secure. (As a final comment, the value of the "secure" is usually referred to as "s-on" or "s-off")