It’s not a simple question, experience and hand-on are the first requirement.
In a really general way, try to use
“pinch roll test” (google) e muscle palpation for hard isolated knots, alteration of connective tissue, and check always the other side.
Self rubbing is an excellent alternative, but much time peoples like tools.
Oh I forgot to comment on this. Generally a good idea but there are cases when you would want to go away from the heart like if you were trying to decompress a joint.