how to want to know the balance of a craft?

@Gozinya thank you

One additional note that may be helpful: there are three different "eyes":
1) Red: center of mass
2) Blue: center of lift
3): Yellow: center of thrust
For aircraft, try to line up the center of mass (red) and center of lift (blue) as closely as possible to make a stable craft. Otherwise, the craft will constantly want to flip around the center of mass. Good luck!

@Fachri789 👍 then you got the answer

@Anand yes

@zeropol thank you

On the building editor it is the last button, with an "eye".
Red is center of mass.
to be sure

Are you talking about center of mass?