Just install it. If it was already installed, it won't hurt anything... and if it wasn't, then it will be.
Note: for SBS, there are 4-5 service packs that should be applied... Sharepoint, SBS itself, SP1, and a couple of others I am not currently remembering.
You might be better off downloading and installing Service Pack 2 instead - but READ the instructions first - there are catches for SBS you may need to install other service packs before it.