Find the slope of each line
Compare the slopes
If the slopes are negative reciprocals, the lines are perpendicular
For slopes in the form (m_1) and (m_2), check whether (m_1 cdot m_2 = -1)
For equations in standard form, rewrite each line in slope-intercept form if needed
For horizontal and vertical lines, a horizontal line is perpendicular to a vertical line
Verify that the lines intersect at a right angle