The easiest and safest way, is to look at the diaphragm itsself. If it is moving outwards when the bass hits, then it is wired correctly.
If it is moving inwards, then it is wired incorrectly.
Another tip is to listen to the bass output. When it is wired incorrectly, the bass will be weaker.
If you have a volt meter you may check the wires (not hooked up to the speaker) one at a time. The positive wire should give you a signal (when the HU is playing) and the negative one will not.
If you do not have a + or a - labeled on your speaker terminal, then label the wider one (+) and the narrow one (-). I hope I am correct on this one. If anyone knows for sure, please correct me if I am wrong.