If you’re doing client work there — and you may not be! —- you’re responsible for the portion of the income earned in that state (you have “nexus,” in the parlance — I’m less sure about merely having presence in a state even if the work isn’t generating company income, but in some cases I think that’s treated as nexus as well). In those cases, your company should send you state W-2s for your income; most decent consulting firms offer tax prep services if you end up working in a lot of state and international jurisdictions with tax consequences.