Used items donated to a charity may be listed on schedule A at their fair market value. Fair market value is what a willing buyer would pay a willing seller in a free and open sale and considering the items current condition.
Intuit has a website called itsdeductible.com which allows you to enter item donations during the year. You can then download those entries into TurboTax when you prepare your tax return and it’s deductible will assign values. However, if you are audited, you will need to show the IRS that the its deductible values are reasonable for your area. The best place to find thrift store values for used clothing and other items is to look on the shelves of the thrift store.
The CARES act created a special front page charity deduction of $300, but this only applies to donations of money, including cash, credit card, and check payments. Non-cash donations must be listed on schedule A and you will only benefit if the total of your itemized deductions is more than your standard deduction.