Systematic reviews strive to minimize bias in all aspects of its methodology. Cochrane defines bias as a "systematic error, or deviation from the truth, in results." Biases can skew the results of your study, such as by over- or under-estimating the effect of an intervention. Bias does not necessarily mean bad intentions on the part of the primary investigator; bias can be unavoidable due to constraints on how the study could be designed.
It is impossible for a systematic reviewer to know for sure whether bias has affected an included study. But you can assess a study's risk of bias.
MECIR C52 Assessing Risk of Bias (Mandatory) - "Assess the risk of bias for each study result contributing to an outcome in the ‘summary of findings’ table. For randomized trials, the RoB 2 tool should be used, involving judgements and support for those judgements across a series of domains of bias, as described in the Handbook." (Cochrane Handbook, MECIR Box 7.1.a Relevant expectations for conduct of intervention reviews)
Popular risk of bias tools include the following:
Find more risk of bias tools for specific study types, including observational and qualitative studies, at the following websites: