I have developped somewhat of an "expectation" on every job i get. What that means is i base my test spot on what i think that paint on that car will respond to given the level of correction the customer wants. That answer does not help you answer your question but i started out by getting a feel for paint hardness reputations from you guys here. I filled my head with what the general scales and experienced opinions are per manufacturer and then, when confronted with them, tested out the validity of that "reputation" (also being careful to avoid the cases of atypical conditions...this "should" be soft but ain't budging). I once had an Audi that i could've used a hammer and chisel on! Sure enough...hard as diamonds. Then i remember the first black Honda... i had to whisper towards it in loving tones to persuade the swirls to lay flat. And there are incredibally wide variations in between and many times even some variations at spots on the vehicle you have in front of you.
Basically, for me, i got used to the reputations many paints have and then do maybe a 1'x1' test spot on the hood with whatever i thought might be most effective. You eventually get much better at evaluating what you think will be the best product, pad, machine, speeds etc. I also use a limited number of products and have gotten used to the working characteristics of those few products. I have found i have developped enough variations to the effectiveness by varying pads, speeds, machines etc.
I should mention that i have also done a test spot or two that yielded too much removal, too aggressive an effort to level the paint. Again...the beauty of a TEST. And by keeping it down to a small size i can correct my approach without leaving a glaring patch of "over correction".
Again, i remembered the general manufacturers hard/soft reputations and then made an educated guess then confirmed or denied my approach through trial and error. After a few years now i've gotten better and faster with my test spots.
I will say that you guys with more experience than me are incredibally reliable for guidance on what might work best for a particular paint. For that i thank you all.
Edit: whoops...was typing out my post and hadn't fully read all the replies so far. I basically said what RTexas said and he was spot on. Doesn't surprise me...he is one of those "pro's" i have learned a lot from!