how do you know the data in any research you are doing is valid?

Answer 1

You gather lots of data in as many different ways as you can. You compare your data to other people's data. You let other professionals in your field scrutinize your data and your techniques. If after repeated testing and vetting the data still stands, then it is probably valid.

You should also search for any possible sources of error. Those that cannot be eliminated should be explained thoroughly.

Answer 2

You don't, until you have enough of it. Remember, two is a coincidence, three is a trend.