How well do radiologists (esp those on call at night) look at cranial ct scans that are sent? do they glance quickly to where it allows for mistake?

It can vary: Just like with any doctor, results can vary. Yes, it is possible that they miss something or overlook something. However, most places that utilize the "night radiologists" have their usual radiologists review the images/films the next day, so really you are getting two readings of the scan. If there is a new or different finding, they will notify you and even occassionally recommend further test.

Equally competent: Radiologists are medical imaging professionals with 13-14 yrs of medical training beyond high school. Your imaging examinations are read in a similar fashion by them 24/7. Due to time scheduling differences, some radiologists (nighthawks) may live in a different time zone but they are similarly credentialed and competent.

Well: The quality of imaging interpretation should be the same whether day or night. No matter what time of day, there is always a possibility of missing something or making a mistake. Radiologists are people and not always perfect.