You want to give your information to an unknnown website that looks like IRS. Turbotax filer has a forum where they raised the same question, but no moran wrote the IRS number was on the fricking letter? 

If you did not have any problem with IRS last year, then there will be no problem this year.
This is the real number to call IRS.

Telephone Assistance for Individuals:
Toll-Free, 1-800-829-1040 
Hours of Operation: Monday – Friday, 7:00 a.m. – 7:00 p.m. your local time (Alaska & Hawaii follow Pacific Time).

If you are smart : It will take over an hour to get connected to person, call and check if they had sent any letter regarding verification.
Otherwise here is the web site. just go to   www.idverify.irs.gov to verify the information.