You must make a good faith effort to determine the cost basis - in this case, it will be the value of the bond on the date it was inherited from the original owner. If they were gifted to you this year, ask the person who gave them to you when he/she inherited them.
Even if you only have an approximate year for when they were inherited, that shows good faith - you can use the average price of the bond for that year.
Once you have an acquisition date (or range), consult your accountant or broker, or go online to fee-based services or free services that provide historical stock and bond prices to find the price as of that date. The investor relations unit of the entity issuing your securities may have historical price information.
If you have a range of possible purchase dates, find the average price of your stock or bond during the date range. If you only have a purchase year, find the average price during that year.
See How to Find Unknown Cost Basis of Bonds & Stocks for additional information and a discussion of the subject.
