Things might turn out to be a bit more complex than you'd first imagined. Collision detection can be quite tricky and depending on what kind of platformer you want to make, it can also largely vary.
This is great article about different approaches.
If you just want to have a intersection detection, then you can do this relatively easy with
SFML:
if(rectA.intersects(rectB))
That's essentially just the easy part, the hard part is handling the reaction to it properly, i.e. what should happen once they intersect? Where do I move the boxes?