With COVID-19, even though fever is a common symptom shared between all types of infections (mild, moderate of severe), the fever recorded during COVID is usually said to be low or moderate grade (touching to 102 degrees Fahrenheit max.) and well-managed with the use of medicines like paracetamols. If a fever persists for over 7 days, the infection is said to be severe or threatening.
With a dengue infection, fever can rise considerably high, (up to 103-105 degrees Fahrenheit) and be accompanied by other concerning symptoms, which may require intensive treatment to get better. According to some reports, the current strain of the dengue virus, DENV-2 is also known to cause more severe intensity of symptoms, including fatally high fever in particular.
The fever seen with a dengue infection can also be continuous and persistent, whereas with COVID, it can come and go. Thus, the very manner your fever sets in with an infection could predict whether the symptoms you have are that of dengue, or COVID-19.