Mention the researchers' credentials. If the researchers have years of experience, or several successful studies done using the qualitative research method, this can help give validity to their current research, as well.
Follow the accounts that were done by the researchers and participants in the study. If possible, use direct quotes and observations from their field notes. This gives less chance for error when using their research.
Use multiple research methods as a way to cross-check the research done using the qualitative research method. If the findings are similar, this adds validity to the qualitative research.
Use multiple researchers. As the old saying goes, "There's strength in numbers." If several researchers are working together and using the qualitative research method, this can add validity to it.
Get feedback from experts in the subject. Have them look over the study. Their approval can help give validity to the study.
Get feedback from the participants in the study. Share the results. If they agree with the results, this again gives "strength in numbers" to the promotion of validity for the research.