If you would like to know how you can distinguish the Nike SB and Dunks, you just need to check out the tongue of the shoes. If you would look at the tongue of the Nike SB closely, you will notice that the logo of Nike SB will be sewed in. If you would check out Nike Dunk, you will notice that only the logo of Nike is placed in on the tongue of the shoes.
If you think that this is the only difference, you should realize that there are still more that are available. The shoelaces should also be taken into consideration. The Nike SB will make use of thick shoelaces while the shoelaces of Nike Dunk will be as wide as the textile laces that are available.