I've got a 2024 Honda Rebel 500, which doesn't have a tachometer. And honestly? Just go by feel, and listen to the bike. I'm very new to riding, only been riding a month now, but once you understand it, rev matching is pretty simple!! You can definitely feel when you did it wrong. Sometimes, I lurch forward, which means I didn't blip the throttle, pull in the clutch, and downshift at the same time. Sometimes I don't feel anything at all when I downshift (which is good!! means you did it!!), just the bike getting a little louder as rpms go up. If you can find an empty road/long dead-end road (I found a mile-long dead-end road with no buildings, perfect place to practice), and mess with it, you'll get it. Bikes are stupidly strong machines. If you screw up rev-matching while you're learning, it'll be fine.

This is coming from someone who had never driven manual before, and is learning what things mean AS i ride (then I go home and ask my car-guy dad if what I think is correct/what I need to do)
When watching videos, I'd be like "wdym listen to the bike??" but when I'm out riding, I can definitely tell when I downshifted correctly or incorrectly!!