How to tell if an acid is a strong or weak acid

For example, HCN is a weak acid, is there a way of knowing that just by looking at it?

I know that a weak acid is one that does not dissociate, or ionize completely.

Yes, there certainly is a way of knowing if an acid is weak or strong, just by looking at its chemical formula.

Strong acids ionize, right? So strong acids must be atoms of the halogen group, bonding with hydrogen. Ex: HCl, HBr, HI

And then, you have to memorize your solubility chart. All nitrates are soluble. Thus, HNO3 is a strong acid.

Luckily for you, there are only seven strongs acids that you need to memorize.
HCl
HBr
HI
HNO3
H2SO4
HClO3
HClO4

Any other acid is a weak acid.