Depends. If surrogate court records exist, you can check the ones down to 1874 on the Rochester Library's web site. That's free, and the Surrogate's Court itself charges 90 (yes, ninety) dollars to search. The library has the A-M 1855 census index online as well, but the Familysearch.org also now has that index and the images online for free.
The newspaper indexes are online and searchable 1818-1897, but they are very large and can take a long time to load if you are not on broadband.
Property records are available at he county court house. I am told that they are online BUT ONLY if you are there; they cannot be searched from outside.
You could try looking in the RGS's church records site: http://nyrgs.org/churchRecordImages/chRecIndex.htm
(although they have done far more than are online) and also bear in mind: http://rmc.library.cornell.edu/eguides/lists/churchlist1.htm...
Since Monroe County was not created until 1821, any marriage before that would be found in church records, possibly in the predecessor county of the Mendon area) in Ontario County. NY state did not require official registration until 1880.