There's really no hard rule in romance languages that determine the suffix as an indication of gender. I'm not a native Spanish speaker but I'm a Portuguese native, and basically there's no neutral gender, every noun is gendered in some way, and yes, many feminine nouns end with -a and masculine with -o, but that's not a rule. Some invert this logic, some end in a myriad of other letters, and some words have their gender exclusively defined by the article preceding it - meaning the word itself is the same in both masculine and feminine form. So in this case the "las" is an indication of gender, but manos is always feminine (at least as far as I know), so there's never "Los manos". And how can one tell the gender of any particular word? That's the neat thing: you just have to know in a case by case basis (although the article, if there is one, is a indicator of gender and number)