How would you ask "did I leave anything out?" in Italian? Like, if I made a list of things and I want to know if I did not include something that I was supposed to. I am thinking "non messo qualcosa?" ma non lo so.
Commonly you will hear a more active form of the expression with the verb "dimenticare"
Ho dimenticato qualcosa?
A more sophisticated (and somewhat quaint/formal) expression that is similar to what you have suggested uses the verb "omettere" (to omit).
Ho omesso qualcosa dalla/nella lista?
As Denis suggested, watch out with the pronunciation between "ho" and "omesso".
As DaG suggested, something similar to what you had in mind which I had not thought about is
C'è qualcosa che non ho messo/incluso?