Secure erase trashed files in El Capitan
You could try a tool like https://www.prosofteng.com/datarescue-mac-data-recovery/ which has some free tiers to recover limited files.

If your Mac OS supports the FileVault full disk encryption, doing that would be far better and let you never worry about secure erase since everything is cryptographically erased without your password. If that's not an option, use Disk Utility and securely wipe all free space on the drive one time.

I'm assuming you don't have an SSD which don't support secure erase - in that case, see here.


Secure delete an erased file using same file