Let's say that you just got the game and you click/tap play,then in the bottom right corner you see that your at level 0. Now, you go to make a plane in SANDBOX, and when you finish you test it out and it works! But when you exit out then you see that you went from level 0 to level 1 because on level 0 you have build one plane, then when you keep making planes you keep leveling up!
We already have a level system here. You are a level white.
And you should already be aware of what your own personal level of experience is, you were there for it.
Steam also tracks your hours playing, so that's an indicator you can go by as well.
Really everything you need is in place, and anything more is overkill.