you can face nail somewhat, walk around and get down on the floor and see where the flooring moves exactly. You can only screw to the joist since thats all you have.
I haven't' tried these but they might be good.
https://www.lowes.com/pd/Squeeeeek-No-M ... ge/4441839 they are screws designed to break off just below the surface. or if you use nails you can set them , and then putty them or if it is a finished floor maybe just use a wax crayon o the right color for hiding nail holes, just rub it in.
if you have access from below you can do more like perhaps driving screws at an angle ( like toenailing) to hit the joist and flooring without breaking through and that would leave no visible holes from above, but you'd need access. If you do have access you can also do this sort of thing:
https://woodgears.ca/home/bracing_diagram.jpg it spreads the load among the other joist and stiffens the floor.
if you drive nails you can try intentionally angling them. if you drive two nails at different angles near each other its hard for them to pull out. sometimes the movement of the floor can actually push nails back out so I'd drive them at angles. then when the floor flexes it isn't just sliding up and down on the nail it is at an angle and that holds better.
you could also try tings like drilling little holes down through the flooring and just partly into the joist and then squirt glue down the holes to try to get some between the flooring and the top of the joist, and then use the holes for the screws after, to pull it tight while the glue is wet.
usually all the boards dont' squeak so youll have to deternmine exactly which ones do and then youll have less nail holes. I'd go minimal and then walk around to see how it is and do more if you have to.
if you want to attack the issue from below I'd see if you can get someone to help find the squeaky spots. you need someone to walk around while the other feels and listens from below.
here's another trick. drill a small hole smaller than your nails at an angle and drive a nail in following the hole. set the nail in fairly deep like 1/8th or so. then drill using the same hole but a different angle. you can put a second nail down and set it. The result : Two nails at different angles, but only one nail hole
- don't do that on casings and things you might take apart again it will lock them together kind of permanently and maybe cause some damage if you want to take it apart again later. But if you do a few that way, I mean,, If you are pulling up that flooring you have bigger issues and destroying the flooring would likely be par for the course at that point.
my attic is like that. 2x4 joist then no shiplap, but its fine. the main floor has super heavy joist. its just the way it was designed, maybe the attic wasn't going to be living space at the initial construction / design phase, I'm not sure.
another option could be to cut plywood strips the width of the joist space and set that up against the floor, then you can drive some screws to hit the joist and others right up into the flooring from below but not breaking the surface.
I dont; mind a few squeaks, it's like having an early warning system