I think it might be a good starting place to compare their personalities. What they like and don't like, what's important to them, how intelligent they are, their lifestyles and home situations, their characteristics, etc. The more you understand their personalities, the more their dialogue will fit who they are, and the more their voice will be 'distinctive'. I wouldn't stress about making every sentence 'identifiable', as this is likely to come off as gimmicky. There are exceptions, as I think some characters are bizarre and unique enough that their voice will be obvious in any circumstance, but for most 'normal' people, there will be situations were individual voice is not likely to show (for example, two characters discussing where they want to eat; its unlikely every line will stand out as this character or that character).
Yes - that is where I started - so happy to hear that was not a bad approach.
Personally, I find my characters' voices show through without a lot of effort, but that there are certain ways I tend to talk or think that bleed through onto all of them. For example, I've begun to notice words or phrases I use in nearly all my dialogue, because it is the way I would speak, but it is not necessarily the way all my characters would speak. I also recently noticed in my WIP that two characters address the MC in very similar fashions, and while it is very in character for one, it isn't really necessary for the other. By editing it so only the one character addresses the MC in this manner, it strengthens both of the characters' voices. If you are able to spot these sorts of things, it might help you make the voices distinctive!