How do I know MA % on this?

Google is your friend. I am in no way a chemist and actually if I took chemistry I probably would have flunked. But I did stay at a Holiday Inn before.

But per an internet search, the modern and overwhelmingly more common name for muriatic acid is hydrochloric acid. Hydrogen chloride is a colorless gas. It can be dissolved into a solvent like water to give you hydrochloric acid. That is, hydrochloric acid is an aqueous solution of hydrogen chloride.

So, that bottle is 31.45% muriatic acid.