You can't.
Facebook doesn't give you a way to automatically get an access token for a user. That user needs to log into Facebook and explicitly give your app permission. The best you can get is a long-lived access token that remains valid for up to 60 days.
Getting that token requires a two step process:
1) Logging into Facebook using either the JavaScript API or redirecting the user to a valid Facebook login URL. 2) Retrieving the short-lived access token you got in step 1 for a long-lived access token.
Once you've got that access token, should your post fail, you know you need to re-authenticate the user and get a new long-lived access token. Your user needs to be online and logged into Facebook for this to work, though it can happen without their interaction.