Federal law permits Goodwill donors to claim tax deductions for donations
of clothing and household items in good used condition or better.
If you’re unsure whether your item is in “good” condition, then consider this:
If you would give it to a relative or friend, then the item is most likely appropriate to donate.

The Internal Revenue Service has many guidelines for determining the
value of your donation.  Some publications which may be helpful are IRS Publication 561 (Determining the Fair Value of Property)
IRS Publication 526 (Charitable Contributions), and IRS Form 8283
(Noncash Charitable Contributions).

Valuation Guide for Material Donations
If you itemize deductions on your federal tax return (using the long form),
you are entitled to claim a charitable deduction for your donation to Goodwill. You will be required to determine the value of these items. 
The list below shows approximate resale values for commonly donated items in good condition.