Statistical test to show a sample represents a population?

In my current research project, I have collected data from 29 patients and reviewed medical records for 390 patients in the last 12 months. The characteristics of the two cohorts appear similar when eyeballing i.e. age, gender distribution, BMI mean. However, I would like to suggest that the results I found from tools used in the small sample would be applicable to the population.
Is there a test I can do to show that the sample of 29 is representative of the clinic population based on the 390 retrospective records collected?