This will be the first rover that is looking specifically for past signs of life compared to other rovers and landers that have looked for current signs of life such as the viking lander program.
The ExoMars rover will be able to drill for samples up to 2 meters below ground. The hope is that it's sophisticated lab equipment will be able to detect signs of microbial activity both today, and any that took place billions of years ago.
The Viking 1 lander which touched down on the Red Planet on 20th July 1976 and performed experiments; where out of 4 experiments looking for signs of life, 3 were negative, while 1 was positive. The positive result has generally been dismissed as a false positive.
"Most scientists now believe that the data were due to inorganic chemical reactions of the soil; however, this view may be changing after the recent discovery of near-surface ice near the Viking landing zone. Some scientists still believe the results were due to living reactions. No organic chemicals were found in the soil. However, dry areas of Antarctica do not have detectable organic compounds either, but they have organisms living in the rocks." - Wikipedia