I'm curious about how does ppl handle this too.
I either use a tempory variables or use an array to store current frame and previous frame values.
Using a tempory variable could be (supposing ur variables are initialized before draw()):

1. float currentVal;
2. float prevVal;
3. float tempVal;
4. void setup() {
5.   size(100, 100);
6.   frameRate(2);
7.   currentVal = random(80);
8.   tempVal = currentVal;
9. }
10. void draw() {
11.   currentVal = random(80);
12.   if (tempVal != currentVal) {
13.     prevVal = tempVal;
14.     tempVal = currentVal;
15.   }
16.   println("c: " + currentVal + "   p: " + prevVal);
17. }
18. void mousePressed() {
19.   redraw();
20. }

prevVal store the previous frame value.
But i'm sure they are clever way to do that.