I don't mean to upset you, but I don't think there is a way to seperate the two. I'm not a pro, so I hope Carlo can help or someone else.. cause I think Carlo was always very into such stuff. He was the genius when it came to language.
But I'm like 90% sure that one can't tell the difference between "letters as numbers" and "letters as letters", cause the other day I was looking at this picture and there was a sentence written under it and I couldn't read this one specific word cause it was a mixture of letters that made no sense (yoth and dallath and something else), so I took a look at the translation and it was a date (the letters were numerals representing a year).
Also, when my mom used to teach me the "numeral system" when I was a kid, she never mentioned "how to know whether this is a number or a letter"..
But like I said.. I'm not 100% sure!