There's a worksheet called "St K-1 Rec" that will show you entity-by-entity how much state-source income Lacerte thinks you have. That's where I start to find data-entry errors.

Sometimes there will be a discrepancy between that worksheet and the actual state tax return, and you have to decide which one is correct and judiciously override. Sometimes there will be a discrepancy between that worksheet and your own outside calculation of the correct number, due to the "-1 means zero" issue.