What testing do science based facts get through to be validated?

[I believe in Harry Potter]

A common question i get from theists is whether or not these information i present them can be manipulated by someone or something or how do we actually know that they are not an bioproduct of competition and many other factors , if science always changes and can be tested upon more tests , even testing the actual tests of the testing methods ( lol ) and things like that. So what is your counter argument for that? How can we be clear for everything we know. Talking about their methods , funding , scientists favoring certain subjects etc and taking in to account.