Turbotax is only calculating a tax for about 30% of my 2018 tax year state refund (vs. the whole thing). How do I tell if this is accurate?

It is based on how much the state income tax you claimed on your Schedule A on your 2018 Federal tax return affected your total itemized deduction amount compared to the Standard Deduction that year.

For example, say your refund was 900, and your itemized deductions were 300 over the Standard, only 300 is claimed as income since that was all that put you over. 

I have attached a worksheet you can use to check the amount reported by TurboTax. 

IRS state refund worksheet