"How do I determine this?"
Typically it is a simple matter to make the determination; either the fees/expenses are all attributable to taxable or tax-exempt income or they are not and an allocation must be made.
The allocation is based on the ratio of tax-exempt income to taxable income. For example, if you have tax-exempt income of $1000 and taxable income of $2000, then one-half of your, otherwise allocable, expenses would be allocated to tax-exempt income and disallowed while one-half of your expenses are allocated to taxable income and are deductible (see attachment).
TurboTax will make the appropriate allocation for you if you enter expenses as being allocable to Both taxable and tax-exempt income.
