Jiāpǔ shàng hái néng chá dào zēngzǔfùmǔ de míngzi.
You can still find the great-grandparents' names in the family genealogy.
Tips
mistakes
Specifically the paternal great-grandparents (your father's grandparents). Maternal great-grandparents are 外曾祖父母. Don't confuse 曾 (great-) with 增 (to increase) - same pronunciation, different character.
grammar
曾祖父 = great-grandfather, 曾祖母 = great-grandmother. Combining 父母 at the end is the standard parents-as-pair shortcut, like 祖父母 (grandparents).
