Fundamentally, you need to prove the value of what you have given, but that can get complicated when you are giving in the form of things. You need an appraisal of any thing you give (e.g., art, a house, an easement, or a service) and you need that appraisal to answer all the taxman’s questions even before he asks them. In short, appraisals can get tricky.
For example, if you give art you need an art appraiser, and if you give real estate your need a real estate professional. However, what if you give interest or stock in a company? Then the appraisal becomes even trickier.
When you are giving less than obviously valued interest, then you might want to consider the advice given in a recent Forbes article titled "It's Hard To Satisfy The Qualified Appraisal Requirement For A Charitable Contribution If You Appraise The Wrong Property." The problem with interest or stock in a company is that the interest or stock is different than the things the company owns. So, 10% of the stock in a company with $5 million in assets is not (necessarily) equivalent to 10% of $5 million. Such is the case, even when the sole purpose of the company is to hold well-documented and appraised assets worth $5 million.
In a "simple case," the IRS likely will provide leniency and determine that such and such an appraisal was "substantially compliant" if not fully so. Nevertheless, the greater the business complexity, the greater the risk of misstep and an attending (and costly) determination of insubstantial compliance.
An appraisal is a very necessary step when recognizing the tax implications of any transfer of property that is more than simply money itself. In the end, when giving to charity or to loved ones it just stings all the more when your attempted good deed is punished.
Reference: Forbes (February 5, 2013) "It's Hard To Satisfy The Qualified Appraisal Requirement For A Charitable Contribution If You Appraise The Wrong Property"