Someone at our house, an adult who is not me (cough, the Rev, cough), has a habit of misplacing his wallet...
and if he is like most of us, he probably has no idea what exactly is in there. Home Storage Solutions 101 has a great list to help you create a wallet inventory so that you are prepared in the event that you lose your wallet.
Here's ideas of the types of information to add to this inventory:
Name of debit and credit cards, plus 800 number to contact in case of loss. (You need to consider whether you'll keep the credit card number on this list as well as the name of the card and contact info. It can come in handy in case of loss or theft, but if the list itself is lost or stolen it can come back to bite you, so just consider what you want to do, considering where you'll keep it.)
Driver's license or other forms of government ID
Checkbooks and bank deposit slips
Health insurance (or other insurance) cards
Frequent flier programs
Gift and loyalty cards
Membership cards, such as for gym, warehouse type stores, etc.
**Quick Hack If You Don't Want To Write It Out**
If the thought of filling out this inventory keeps you from doing it another quick way to do something similar is to use a photocopier and lay your various cards face down and make a photocopy of them. Then, flip the cards over and do it again.
With this method you'll have a copy of what exactly is on all your cards, front and back, that are in your wallet. You can reference these photocopies as well. Just make sure you make some notes on the copies so you can readily match the right front with the right back so you don't get confused.
And obviously, don't store the list in your wallet...