Innovating examining in the direction of promoting technology, and bringing technology into exams is an inevitable trend because it minimizes unnecessary human intervention, approaching the world's examining and evaluation approach. Many universities around the world have been building online training systems to manage learning and organize online exams. This study builds a new monitoring function during the online exam process. A function is implemented on the computer device to detect the temporary exit behavior of candidates. Each time a candidate exits the test, the function will continuously save the screen image of the candidate's device in real time on the server. The images are saved as a basis for determining whether the contestant has cheated or not. The function is practically implemented in the subjects of 23 classes with about 1500 students from the 2021 to 2024 school year at Hanoi University of Education and gives accurate results in identifying cheating behavior of candidates with the rates according to school years being 40%, 19.37%, and 2.9%.