First thing: go into training mode, scoot back (or forward ) a little and find a spot in the background to mark your starting position (example: put your front foot by a rock or the tire of some car or anything that stays constant. This spot should be near your starting position on training mode reset so that you can consistently mark your position versus the dummy’s default starting position. From there you can start scooting forward or backward a single step at a time to inch your way in or out of your attack range to find the farthest distance that an attack will hit. Some attacks will move your character forward a bit, which you can tell by monitoring your foot’s starting position before and after performing the attack.
As for your pokes actually “working” that is a question of frame data and hitboxes. It’s hard to test in training mode, but you can record the dummy executing a poke and stand at whatever distance you want to test. Try starting your own attack at various timings while the dummy is playing back their recorded move and you can test things that way. I don’t necessarily recommend brute force testing with this method too much, it’s more of a tool you can use to test specific things you notice after a match. “I had a hard time poking from X distance against this character, so I will test my attacks out against their attack at various spacings and timings”
In general though it is much easier to feel out a matchup by actually playing it than to try and test pokes in training mode. Footsies is a very visual skill and can be learned on the fly without research or grinding training mode too much.