Depends on what kind of radar. There are two kinds used to identify aircraft, primary and secondary.
Primary radar is the traditional "old fashioned" kind of radar, it uses radio waves to determine the aircraft's distance and azimuth from the station. In order to avoid detection, the pilot must fly low to the ground, or fly an aircraft with stealth abilities such as acute angles on the airframe, or absorbent paint.
Secondary radar is all electronic. The unit on the ground is an interrogator, it transmits a radio signal to the aircraft's transponder. The transponder sends back a replying signal to a decoder on the ground. A computer processes the replying signal, and the end result is the aircraft's identification, speed, and altitude, displayed in the form of a data block on the radar screen. In order for a pilot to avoid detection from secondary radar, all he/she has to do is turn off the transponder.
Commercial pilot with some air traffic control experience