You can connect it and put your hand in front/behind it to see the direction. However the front (pulling air in) is usually where the company logo is and the back (pulling air out) is where the wires are showing.
I think most (maybe all) spin to the left
Make sure that they are getting the right amount of power because some cases have little resistance switches that control the power to the fan (i.e. with the NZXT Phantom when you switch between the three fan 'speed' settings the lights on the LED fans will dim accordingly as a result of less and less power being delivered to the fans) so don't blame the fan performance straight away; unless of course the fan is rated as such by multiple people.