Tea is traditionally made from a specific species of hibiscus grown for the pods: there are some details here https://www.tyrantfarms.com/hibiscus-a-tasty-addition-to-your-edible-landscape-or-garden/

I don't know if you'd want to make syrup out of a random mallow. You might want to first taste the flower (which is apparently safely edible across the genus) and nibble on a few young leaves. Then i suppose you can see if the pods, three or four days after the bloom, are surrounded by a thick petal-leaf like structure (the calyx) or if it's bare, like okra. The calyx of the H sabridaffa is bright ruby red and thick: that's what i made the tea with. Hibiscus syriacus (Rose of sharon) just has pods with out a fleshy covering.

Here's more reflections on edibility: http://www.eattheweeds.com/mallow-madness-the-false-roselle/