Knowing where to drill

I’ve got HVAC in my two car garage, and I’m moving my computer and whatnot into there as like my man cave. The garage is directly underneath the two kids bedrooms, and the router is currently next to the wall on the outside of one of their rooms in the hall. I need to run a cord down into the garage. How in the world do I know when i drill a 1/2” hole into the floor that there’s nothing that shouldn’t have a hole poked into it below.