Maybe are you planning your Data Management Plan or maybe are you designing an experiment. As a researcher you would need to know the previous existing data.

It is possible to access existing data when they have been uploaded in data banks or added to scientific papers as supplementary material:

• re3data.org compiles the repositories http://re3data.org
• ODiSEA records the scientific journals that support supplementary material http://odisea.ciepi.org