The most simple answer is: If you can hear it, they can hear it as well.
If you can't hear it, there's no way to know besides asking them.
In a physical sense, sound is a vibration of air. It spreads in all directions from its point of origin. It can cause objects to vibrate as well (that's how microphones work) and can be spread further by objects that transmit the vibration.
If you stomp your foot on your floor, the origin of sound is the floor where your foot lands. From there, the sound waves spread up to your ear, but also down to your downstairs neighbor and sideways through the floor (which makes your couch vibrate). The path to your neighbor is probably somewhat blocked by their ceiling, but in general that doesn't dampen sound much.
If the vibration of your couch is audible to you, you can be sure it's also audible to your neighbor.
If the rubber mat causes inaudible vibrations, then their ceiling lamp might still swing like a pendulum or the vibrations might make some glassware clank. The safest way to know for sure is to ask.
Another method that involves a lot of guesswork is comparison. What kinds of sounds can you hear from your neighbor? Can you hear them talk, walk around or call someone on the phone? Then they can probably hear sounds from your appartment that are equally as loud.
The risks with this method are:
- Different people have different habits. Some become very loud on the phone although they can hear quiet sounds just fine.
- Sounds of different frequencies are dampened differently. At the same physical loudness you might still hear a high pitched sound while a low pitched one is inaudible, or vice versa.
- Your floor is closer to their ears than their floor is to yours. They can hear things that happen on your floor much louder than you can hear things happening on their floor.
If your neighbor complains about the noise or vibrations, it might help putting the treadmill in another spot, preferably in a corner or at least very close to a wall. The structure of a house is stronger where walls and floors meet, which can at least avoid interferences.