That only comes with experience. Knowing when a piece is working or not is up to you, and most students struggle with this in the beginning.
Best advice - if you start feeling doubts, set it aside, start something else. Revisit the piece later, and decide if it's worth continuing.
This is one way a teacher helps, either helping you develop something so you don't fail, or giving you a nudge that it's failing.
This forum can help, by posting work and asking for advice.
The skill here is having a finished endpoint in your mind, and working towards that. That's also something you build towards, and why beginning courses start with simple objects, and you learn how to build towards a finished piece over time. Most beginners also work too far ahead their skills a lot of the time.
If you are feeling it's not good, then put it to the side. Your instincts are probably correct. A lot of art becomes instinctual with experience, yoy just don't have enough, so you question it, or try to plow past it. Start listening to your instincts, it's a skill/muscle that has to be trained along with everything else.