Question

Is aluminum sulfate ionic or molecular? How do you know?

Answer 1

Ionic. Aluminum is a metal and Sulfate is not, so it'll technically be Polyatomic.
I'll teach you an easy way of looking at this: Al (3+ charge) and SO4 (2- charge).

Answer 2

When you have a positive (cation "+") metal and a negative (anion "-") non-metal, and you put them both together, it forms an ionic compound.

Metal + Nonmetal = ionic
NaCl = Ionic
Na (Metal) + Cl (Nonmetal) = Ionic
Hope that helps!