I just released a video on the YouTube channel that shows you how to self-correct your harmony homework: https://youtu.be/Zn-nEH35m5Y. There are 3 main ways to catch parallel 5ths & 8ths in your exercises: 1) Look at the soprano and bass uniquely, and pay particular attention when they are moving in the same direction. 2) Bass moves by step: other voices must move in opposite direction, except for a deceptive cadence, in which the leading tone resolves UP TO THE TONIC. 3) Doublings: when you double the third or fifth, and then go back to a standard doubling (root), there is a high propensity for parallel 5ths and octaves. Watch the video in full for more details. Have a nice day! -Barney