Police verification will be conducted. UPSC will send information to the police station of your area to enquire and give a certificate. The police will verify the facts and submit the report. Generally, police will come to your house and ask about any pending cases in the court and they may enquire about the candidate in the surroundings and make a report and will forward it to UPSC. Nobody will check emails and social media postings, Generally. But they will also check any cyber cases against the candidate also.
UPSC will also contact the persons whom you have referred to in your application as references and get the details about you.
In addition to that, they will check all original certificates of the candidate pertaining to Date of Birth, Educational qualifications and marks list, transfer certificate, conduct certificate, caste certificate in case of reserved castes, certificates which are submitted by you as proof for claiming the reservation. Aadhaar etc before giving you the final appointment letter.
Medical tests reports by a doctor referred by them or a qualified and recognised doctor are also required and the doctor should qualify you as medically fit.
Then you will be given the appointment letter.