Hire an appraiser
While it can be tempting to get the beneficiaries together and distribute all items based on what they want, every item in the estate is going to have a different value. It is your job as executor to ensure that assets are distributed fairly. Part of doing your job properly is to prepare for distribution by valuing the estate so distributions can proceed properly.
You are not expected to do this valuation yourself, though. Professionals should be consulted to appraise everything from real estate to antique furniture. If, in the course of looking through important documents, you find information relating to the value of particular items in the estate, you should give a copy to the appropriate professional. You also should keep a copy in your filing system in case you need it later.