As far as i know they dont, but they make sure all documents uploaded are valid and authentic. They manually verify eca and ielts provided with applications. I believe they also try to verify bank letter and statements (bank may not disclose any personal information, but they can verify if the letter was issued by them using a reference number). If they have any doubt about employment reference letter, they contact the employer and may even schedule an interview. If they are doubtful about the authenticity of the pcc, i believe they outright reject the the application.