deceased member baptism date

how could i know a deceased member baptism date?

Maybe try familysearch.org if it has been over a year since the member passed away.

Exactly right. When the death of a member is recorded, basic vital data and ordinance data are transferred to FamilySearch.org. Recently deceased members often have multiple duplicate records in FamilySearch.org. You can tell which record came from the membership record by looking at the ordinances listed. If the baptism date is prior to the death date, then the information came from the membership record.