Being that it is spring, you might find yourself engaged in some spring cleaning these days: washing the windows, scrubbing the floors, cleaning out your bank account to buy more Claritin, etc. You might also be clearing out the attic or basement, which might turn up some forgotten treasures. In fact, some of those treasures might be real assets -- as in, stocks. Here's an email we received at Fool HQ:

My mother-in-law has 22 shares of Mid-South Insurance, which was bought out by Trigon Insurance, and then, I think, by Blue Cross/Blue Shield. The certificate is about 20 years old. How do I go about finding the current value of these shares, or how to cash them in?

We couldn't count the number of emails like this we have received over the years (well, maybe we could, but that would leave less time for foosball). Keeping stock certificates at home, rather than leaving them in a brokerage account, is like keeping cash in your mattress instead of a bank. It's too easy to lose or forget about the shares -- or, as in this case, the shares lose you.

Normally, a corporation will keep its investors up-to-date on sales, mergers, stock splits, etc. In this reader's case, it sounds like the corporation may have mislaid mom-in-law's address somewhere along the way. (That's a polite way of saying she probably forgot to send in a change of address!) There is also the possibility that the shares became worthless.

There's an easy way and a hard way to establish the value of old stock certificates. It's just a guess, but we suspect that you might prefer the easy way: Deposit them in a brokerage account.

Even if you don't have a broker, you can open a brokerage account and use the shares to fund it. Since you can't trade until a value is established for the shares, your broker will do all the work for you.

You'll probably want to use a discount broker with a low or no minimum account size requirement since you don't know what the shares might be worth. Open the account and send in your shares by registered mail. The broker will consult the appropriate oracles necessary to establish the value of the shares. It's a case of aligned interests. You want to sell the shares; your broker wants you to sell the shares, buy something else, sell it, buy something else, and be so pleased with his service that you give him all your bank accounts, the contents of your children's piggy banks, and the spare change under your couch cushions. OK -- you have partly aligned interests. But you can use his eagerness to work on your behalf.

If you want to get an idea of the value of the shares before you deposit them, or if you don't want to sell them, you can try doing a Web search on the company's original or subsequent incarnations. If any of these companies are public and have a website, they will have a contact number for the investor relations department. That's a good place to start. Another source is the transfer agent (usually a bank) listed on your stock certificate. There are also books in the reference sections of most larger libraries that can help you track down old companies. It can be very difficult to track down a company that has changed identities along the way. It can be fun, though, if you get into Sherlock Holmes mode.

Be prepared. The sad fact is most old stock certificates are worthless. But there are enough lost treasures out there to make it worthwhile to check them out. Plus, some certificates are collectibles, so you might be able to sell your shares on eBay (NASDAQ:EBAY).

Got an old stock certificate in the bottom of your sock drawer? If you want to check out its value before depositing it with a broker, see our Foolish FAQ on Stock Certificates (bottom half). And visit the Old Stock Certificate Mysteries discussion board. Good luck!