The way I remember strong acids vs. weak acids:
I memorized these Strong Acids: Perchloric acid HClO4, Hydroiodic acid HI, Hydrobromic acid HBr, Hydrochloric acid HCl, Sulfuric acid H2SO4, Nitric acid HNO3, H3O+.
Any acids besides the ones above are WEAK ACIDS.
Ex: HCl is on the Strong Acid list so it's a strong acid.
Ex: HIO3 is a weak acid because it's not on my list.
Since there aren't that many strong acids, it's best to have them memorized to know which are strong and which are weak. Later on, I'll probably learn exactly what makes a strong acid strong..but until then..this is what helps me on the tests.