As Three Dog Night suggested, I would look for a manual. There are differences, and especially I think between drop down bobbins as opposed to the side loaders, but also as proprietary differences just because...
I think all the machines I've come across have a curved and a flat side in the needle holder. Might be hard to look (maybe a little mirror?) but I can feel the needle slip into the space correctly, especially as you tighten it down.
If the machine is new enough to have a threading assist or automatic threader, that can give an additional clue.