Most people use a stud finder to locate studs through drywall but the same principle applies when looking for a joist in the floor.
How to find floor studs.
Kneel down on the hardwood floor and gently tap the head of the hammer on it paying special attention to the sound it makes.
This 3 in 1 stud finder offers a built in leveler and ruler for added value.
Use a long small 1 8 or smaller drill bit and go under the house drill up through the floor as close as you can get to the floor beam.
Go in the house and locate the drill bit now you know where the floor beam is and the very small hole is negligible.
I also use this method in my sheet rock ceiling to locate the ceiling walls in the attic.
Turn the stud finder on place it on the floor and move it.
It is very easy to operate and requires only the use of a single button.