Ascertain the sample size. A study of 50 teachers at a medium-sized high school should include interviews with more than just one or two teachers. If qualitative research relies on too little data, the experiences of only a small number of interviewees might distort the data. For example, interviewing only the two gym teachers might give a distorted perspective on the question of how physically active the children are at the school.
Look at the interview questions or observation methods. According to the National Science Foundation, high-value qualitative research will gather its data in an unbiased way. For example, researchers will not pose leading questions to interviewees. If asking for teachers' views on corporal punishment, a questioner should ask "What are your views on corporal punishment?" rather than "Don't you agree that corporal punishment is a bad thing?" Similarly, if a qualitative researcher wishes to observe classroom behavior but gives kids sugary candy before each class, the results are questionable.
Determine whether the context is relevant. For example, if a researcher wanted to investigate the effects of drunk driving on driving performance, it would make no sense for the researcher to watch sober people driving, or drunk people driving on a fake road far away from traffic.
Check that all the relevant data for each conclusion are clearly displayed. Unless you can see the data used to draw a conclusion, you cannot trust the conclusion fully. Trustworthy qualitative research will share with you the data upon which it relied.