I want to know if there's a way to find out if several points form a straight line. I could of course use y=kx+m but i don't wanna solve it that way. I could check if the points have the same y-value but different x-values. But how do I check if the line is a diagonal?

e.g let's say I have theese points: P1(2,1), P2(4,3), P3(6,5), P4(8,7). I can easily visualize 'em and see that they form a straight line, but how can I solve it without drawing them?