There are some experiments and projects that are searching for extraterrestrial intelligence.(SETI)
I want to know what signs exactly are they looking for? Do they just look for monotone sinusoidal radio signals?
This is a question of philosophy, clearly.
Basically people are looking for every type of signal which supposedly can not occur naturally/inartificially. The general assumption is, that e.g. prime numbers can not be found in any significant role in nature thus every occurrence of them must come from some kind of thinking process-intelligent life.
On the contrary, it is hard to identify such criteria. Humans can only judge based on what they know, so one could easily miss any signal which does not comply with human criteria.
The wiki page on SETI, for which you provided a link, goes a bit more into the details. It is also worth the effort to have a look into the work of Carl Sagan. You might know his novel Contact. But he also wrote scientific / philosophic texts about this topic. Most interestingly, he also advocated to 'calibrate' instruments on space probes intended to find life on other planets (like Mars) against Earth as a test subject first. Some of them failed to verify life on Earth. (In this context, people assumed for a long time that life in space had to look like life on Earth and therefore searched for e.g. spectral signatures of hydrocarbons.) It is a similar problem with the search for criteria in SETI.
They basically look for unnatural signals, for example a carrier signals (one narrow band which is very energetic against the noise background is a good example).
The problem with this approach is that you can't differentiate between noise and highly compressed data (so the propability to catch usual communication data is very very impropable).
Another problem can be other communication channels, for example communication with neutrinos is very lucrative because it can only absorbed by black holes and neutron stars.
Searching for radio signals is probably the most mainstream method.
Transit Photometry is another method that has been in the news lately. Theoretically, it should be able to detect alien megastructures around stars. One candidate has already been identified.
Optical searches for laser communications has also been attempted.
Spectroscopy is a method for which I hold high hopes. When perfected it will able to tell us the composition of the atmosphere of other planets and thus could reveal signs of life. While this is not exactly SETI, we might be able to detect the presence of pollution which could indirectly point to intelligence.
More info on the different methods can be found here
The person that actually developed the algorithms for searching distant radio signals for signs of intelligence is Kent Cullers (portrayed as Kent Clark in the movie Contact). He worked at NASA Ames and the SETI Institute until he retired in 2005. Although blind since birth, he has a contagious enthusiasm which you pretty much need in his line of work. Any papers by him are a good place to start.
Unfortunately, many papers cost money unless your institute's library subscribes to the journal. I did find a public source in a proceedings that you can find from the search below.
Since the link from the search is about 300 characters long, the simplest way to find the work is to do a Google search for: Proceedings of The Search for Extraterrestrial Life: Recent Developments: Kent Cullers
For info about Kent, see: http://www.seti.org/kent-cullers