How do I know if lines are perpendicular

You can tell if lines are perpendicular or not by seeing if meet in a right angle. A right angle is like a square, and the lines would have to be 90 degrees.

One example is when the two lines (horizontal and vertical) meet each other at the ends.

(You MUST make sure that they are directly horizontal and vertical. If not, they will either be acute or obtuse).

I hope this helped!

okay wait before i answer, is this for your points on a graph?