If I want to use a non-contact infrared thermometer to read the temperature of each cylinder at the exhaust port during idle, how high of a reading should the thermometer be able to handle?
According to this website from the University of Washington:
Manifolds and/or exhaust pipes on some vehicles can reach 1200 degrees F. It is rare to find temperatures this high in normal operation.
If your laser thermometer can read up to at least that high, you should not have any issues.
EDIT: As pointed out by @Zaid below:
... if a cylinder is running lean it can drive exhaust manifold temps up to 1600 F.
This would be a very lean condition, but is possible.
I just checked the exhaust manifold of my 1934 Buick with a straight 8 after about a 15 minute drive. The temperature about 4 minutes after shutting of the engine was about 450F degrees, checking with an infrared laser thermometer. It's about 75 degrees outside. The engine temperature while running was about 140 to 150 degrees, as indicated by the original, but restored, dash temperature gauge.