Unfortunately there isn't an existing easy way to find out if your SSN has been used to file a tax return. When you file your taxes via efile[0], the IRS system will reject the tax return if a tax return with the same SSN has already been filed for that processing year.

If this happens to you you're forced to file by mail.

[0] Doesn't matter who you use to do your taxes. Even an accountant that has access to e-file.