You want to WAIT until if you put pressure on your fingers (not your whole weight) you do NOT push down into the concrete. (it is kind of an experiance thing) but I would say if you can put a brick on the concrete and NOT leave a mark you can broom it.
TIP
Using a shop broom PULL it towards you slow and consistent. Pick it up and lower it slowly on the form on the opposite side of you then repeat on the next strip over.
If the strip is TOO wide to reach with a single handle tie a ROPE onto the broom FROM BOTH ENDS. Have someone on the other side 'pull it back' then you pull the broom to your side.
If done properly you should get little concrete 'crumbs' on top of the floor. LEAVE THESE ALONE. Do not worry about them until AFTER the floor is completely cured. They will broom off VERY EASY.
IF you broom and find out it is TOO WET YET. Just bull float or trowel them back down and wait a little longer.
IF you are NOT getting ANY 'broom effect' I would try:
#1 PUSH HARDER - apply down pressure on the broom trying to get the desired effect.
#2 VERY LIGHTLY MIST some water on the floor. YOU MUST BE VERY CAREFUL TO ONLY GET A VERY FINE MIST on the floor. Then try #1 again.
Hope this helps