Harry kept his eyes skinned for a sign of Hagrid all the way down the slushy High Street, and suggested a visit to the Three Broomsticks once he had ascertained that Hagrid was not in any of the shops.
There are no more uses of "ascertain" in the book.
Show samples from other sources
A committee was formed to ascertain the level of public support for a new bike path.
The slaveholders have been known to send in spies among their slaves, to ascertain their views and feelings in regard to their condition.
Douglass, Frederick -- Narrative of the Life of Frederick Douglass An American Slave