The validity of a study explains the extent to which the questions asked measure what they are supposed to measure. In other words, are the questions asked unambiguous. Could the respondent have interpreted the question differently than you had intended it? And regarding the questionnaire as a whole: do the questions asked in the questionnaire answer the research question.
By challenging a questionnaire, you can find out if it is valid. You can do this by presenting the questionnaire to a test person and having them think out loud while completing it. Another possibility is to present the questionnaire to a test group and have them ask questions as soon as a question is unclear. It is essential that your test persons/group resemble your research population. So if you want to present the questionnaire to young people aged 15 to 18, your test persons/group should consist of young people aged 15 to 18.
After this test, especially with a test group, analyze the answers and ask for advice from a fellow researcher. Are the answers consistent? Is the logic within the questionnaire correct? Are the results consistent with similar studies?
It is more important to test, adapt, and retest with newly developed research instruments. Especially if you want to measure more abstract concepts such as attitude or development, extensive testing is needed to get a valid questionnaire.
When using an existing questionnaire, you should take a moment to see if it has already been tested for comprehension and consistency.
In addition to validity, the concept of reliability is often discussed when conducting research. Want to know more about this? Also read my blog about reliability.