Through independent dating of a variety of different radiometric isotopes that have known decay rates. Both from different samples are taken from the same source, as well as different isotopes on the same source, if possible. But don't just listen to me, read what Dr. Roger Wiens has to say in:
Radiometric Dating, A Christian Perspective
There are other methods of establishing and correlating dates that have been used to firm up history out to about 100,000 years without just simply relying on radiometric dating.
Finally, Astrophysics provides still more evidence of an ancient universe, in case all of the other evidence isn't enough, but that's another post.
And how do you know those radio isotopes weren't created at a half-life? How can you be absolutely sure of this?