The www.epaveldas.lt website has a list of its online database of registers of Roman Catholic parishes in Lithuania [clicking on this link downloads the list to your computer as a .pdf document].
The list is in alphabetical order by town name. What you are looking for on this list is the name of your ancestor's town, which will be the first word (or sometimes first two words) in the second column. If your ancestor's town isn't there, there are no online records; if it is there, then at least some of the parish registers are online.
The list is in Lithuanian, and changes from time to time as additional parishes are added. Here is a guide to translation/ interpretation of the list.
The list is titled "ROMOS KATALIKŲ BAŽNYČIOS PARAPIJŲ, KURIŲ SKAITMENINTOS METRIKŲ AKTŲ KNYGOS PASKELBTOS PORTALE WWW.EPAVELDAS.LT, SĄRAŠAS", which means something like "List of Roman Catholic Church Parish Registers Published on Website www.epaveldas .lt".
The first column (Eil. Nr.) is the item number on the list.
The second column (Romos katalikų bažnyčios pavadinimas) contains the name of the town in which the church is located and the name of the parish/ church. The first word (or sometimes first two words) is the name of the town written in the Genitive Case. This is followed by the parish name (often beginning with "Šv." or "Svč.") written in the Genitive Case, typically followed by the phrase "Romos katalikų bažnyčia" [Roman Catholic church]. The Term "Šv." typically means "Saint" and refers to the saint after whom the parish is named, except that "Šv." can also mean "Holy" or "Blessed". The term "Šv. Kryžiaus" means "(of the) Holy Cross". The term "Švč. Mergelės Marijos" means "(of the) Blessed Virgin Mary", and the term "Švč. Trejybės" means "(of the) Blessed Trinity" or "(of the) Holy Trinity". You don't need to use the parish name to access the records, but you do need to know the town name (in the Genitive Case). The town name in the Nominative Case (the name we think of as the "actual" town name) will have the same stem but a different suffix ending (e.g., "Tverečiaus" (Genitive Case) becomes "Tverečius" (Nominative Case)).
The third column (Bylų chronologinės ribos) gives the year range of records that are available in the database. This doesn't mean that all records are available for all years; you have to actually access the database to find out what is and isn't available.
The fourth column (Fondų Nr.) gives the "Fondas" [stock] number used by LVIA to catalog its inventory. [This is usually the first number used in a "source citation" if you reference the record.]
The fifth column (Partneris) lists the partner.
The sixth column (Bylų kiekis) gives the quantity of parish registers that are in the data bank.
The seventh column (Vaizdų kiekis) gives the quantity of digitized images that are in the data bank.
For a description of how to access these online RC parish registers, go to the View Online Lith RCC Registers [Preferred] or View Online Lith RCC Registers [Newer] page of this website. [Even if your ancestor's parish records aren't online, you can still review the online records to get an idea of the typical content of parish registers.]