A
password will not do. Any
mind reading technology could easily scan your
memory for the password, give it to the
devious fraud who is claiming to be you, and then he would be home free for doing all sorts of dasterdly deeds.
The solution lies in DNA testing. It's the only way to know that the person from the future that you are dealing with is actually an older version of you. The person from the future would present a DNA scanning machine (he has one because he's from the future, and if he can travel through time he can have the clarity of mind to bring along his portable DNA scanning machine.) You place a drop of blood in one hole of the machine, he places a drop of his blood into the other (watch him, you don't want him to use any fake blood sacks à la Gattaca), and you listen to the machine whirl away. If the green light comes on, he's you from the future. If it's the red light, beware, because he might have a ray gun or lazer blaster (or some other futuristic weapon). In that case, you will be on your own in a battle between technology and wits.
If the light is green, and you don't trust the machine, you can test it out on other life forms. A blade of grass, a cat, a squid, etc. Use your imagination. You are trying to fool a machine potentially endowing a hive-mind intelligence quantumly linked through an einstein-rosen bridge to the interstellar faster than light communications network of the future.
Once you've proven to yourself that the machine is real and not rigged, and the light is green, you're good to go. I leave to personal taste whether or not to accept a clone as a future version of you. That seems like a decision best left up to the one faced with the problem. Just beware of clones that have been created and brainwashed to want to destroy you. They don't have your best interests in mind.
I hope I have shed some light on the situation. Beware of password-based schemes to protect yourself from practical jokes. They are bound to fail once telepathic machines are created. DNA technology is the future. It will make your life much easier.