Both Uses

ascertain
in
To Kill a Mockingbird
(Edited)

When the new wore off his grandfather's watch, and carrying it became a day's burdensome task, Jem no longer felt the necessity of ascertaining the hour every five minutes.
p. 69.1

ascertaining = determining
After consulting a tree to ascertain from its lichen which way was south, and taking no lip from the subordinates who ventured to correct him, Colonel Maycomb set out on a purposeful journey to rout the enemy and entangled his troops so far northwest in the forest primeval that they were eventually rescued by settlers moving inland.
p. 296.8 *

ascertain = determine

Definitions:

(1) (ascertain)
find out, learn, or determine with certainty -- usually by making an effort
(2) (meaning too rare to warrant focus)