What is the distinguishing characteristic that radio seti experiments are looking for when searching for signs of extraterrestrial intelligence?

The distinguishing characteristic that radio SETI experiments are looking for when searching for signs of extraterrestrial intelligence is the Radio emission that extends over only a narrow band of frequencies. 
The search for extraterrestrial intelligence is a collective term for scientific searches for intelligent extraterrestrial life, for example, monitoring electromagnetic radiation for signs of transmissions from civilizations on other planets. 

Not sure what you mean by this question