On Yahoo answers, if My response has been picked as the best answer, how would I know this?

There's several ways:

a) Check Emails
When you get awarded sent an automatic email entitled "Yahoo Answers: Your answer has been chosen as the best answer". It will include a link in the mail so you can see which one it is

b) Check your profile by "Best Answers"
- Go to your profile name listed at the top right of the page
- This takes you to your profile page and it defaults to your questions
- Click on the % sign on your profile and it shows you only questions where you have been awarded the Best Answer

c) Check Notifications
- Above your Profile name at the top right of the page is "Notifications". Click this
- This takes you to a page where you can see your recent activity
- You will see a link "Congratulations! You won best answer for.." and this shows you that you have just received a Best Answer.

Unless you have changed your settings, you will get an email telling you.

You can also click on notifications in the main page, and see things like this:

"Congratulations! You won best answer for Insurance rate for teen?"

Your points increase. The other thing is that if you subscribed it you can know it by e-mail as well

you get a notification

If someone picks you as best answer you should automatically get an email from Yahoo Answers in your inbox. It says something alone the lines of "congratulations" and the question you answered should be displayed at the bottom of the email. You also get a ten point increase for being chosen as best answer.

You get an email; you can go to your profile page and see all Best answers you've given.