as opposed to reliability which shows that the test is constant, validity shows that the test is valid, meaning that it measures what it is supposed to measure.

For example, IQ should test something along the lines of how smart you are, not how happy you are.

There are many ways to check the validities.

Face validity - Does it look like it measure what it is supposed to measure? is the question here.

Content validity is Are the contents in the test comprised of things that are legit? like statistically based questionnaire and symptoms?

Costruct validity is Are the contents correlated to the already exisiting test materials that measure the same thing? 

Among Criterion-related:

concurrent (Does it measure the condition of the person NOW?) / predictive validity (Does it predict what that person will become?)

I hope this helps. Dont stress too much about the details at this point. Really understand the bigger picture.~