You can test now but your result may not be accurate. It can take up to 14 dpiui for a positive result. If you test now and it's Negatve, don't be bummed yet it can still change into the BFP you're looking for!
It differs with different individuals; there are some who can get pregnant after less than 14 days from the time IUI was done and you should do a test after two weeks. For more information you can visit http://www.fertilitystories.com/iui.htm.