Are you supposed to make a line call instantaneously or
can you take a second look?
The Code of Tennis says you can take a second look:
"When time and the court surface permit, a player
should take a careful second look at any point-ending placement that is
close to a line. Calls based on a "flash look" are often inaccurate,
and the "flash look" system has a high probability of being unfair
to an opponent."