Answer
Strong acids: $HCl$, $HNO_3$, $HBr$ and $H_2SO_4$.
Weak acids: $HF$, $HCOOH$, $NH_4^+$ and $HCN$.
Work Step by Step
The only way to know if an acid is weak or strong, is looking into a textbook, searching the internet, or memorizing the 7 common strong acids:
$H_2SO_4$ , $HCl$, $HBr$, $HNO_3$, $HI$, $HClO_4$ and $HClO_3$.
