One way to do this is to define the core elements of your game when designing it, and separating them from the more advanced ones that perhaps are awesome, but are not really required for the minimal/core version. Once you've done this, estimate how much time it's going to take you to have it working, even if sort of buggy. Set a deadline for everything to work, and since this is the minimal version of your game, it shouldn't take that long; if it is, then you might want to rethink your idea (since it might not be achievable within 48 hours) or try to separate again the core elements from the extended ones. You should aim to have your (very basic) game working by then.
Ideally, at this point you have something you can show someone and have them test it. Make sure you tell them this is only an initial prototype and be very clear on what you want them to test (movement feel, difficulty, controls, jumping force, etc.). Even if it's just a square jumping around in a 2D plane, people can tell whether controls/movement/jumping feel good or not, so use this to your advantage and playtest your game!
After this, you can keep adding stuff and iterating on testable versions of your project. Now, because it's a jam, I know every developer is frantically making changes to the game all the time, but I do believe these small playtests give you a good baseline on what feels good and what doesn't.