Newbe Dev Stack

Answer : The ability to add friends via the website is a popular feature request (a quick Google search will turn up dozens of threads going back years asking for it), but it has never been added. So as of now, you can only do it through the PlayStation 3, PlayStation Vita, or PlayStation 4 (oddly, you can't even add friends on the PSP). Still can't add friends from the website, but you can use the official PlayStation iOS/Android app : Very old question, but in case someone is googling this, now you can do it: https://my.playstation.com/ and navigate to "Friends". From there you can now search, add friends, manage requests and everything else you could only do in the app before.

Answer : The only way to check code is by trying to redeem it. After entering the code, if the code is valid, there will be a confirmation step, and you can cancel the operation. If the code is invalid or expired it will be rejected immediately with an error message.