The part number of the board is on the edge of the board, or written on a sticker. I think the p/n for the 10mhz board is 755-057xxx but IGT made several variations over the years. Another 10mhz variation has p/n 755-0110xxx.
There are several visual differences but the easiest way to tell the 10mhz and 16mhz mpu boards apart is by the round blue volume control knob - the 16mhz board doesn't have one! The 10mhz mpu has the round blue volume adjust on one side of the board and it sticks thru a slot in the metal mouting tray so you can adjust it without having to remove the board. The 16mhz mpu does not have this manual volume adjustment, the sound level is set in the TEST menu system.
Although the 16mhz board is viewed by IGT as a new & improved version many people like the 10mhz board better. On most S+ machines you can use either board, but there are a few that require the 16mhz version mpu. If you swap one version mpu for another you may not be able to use the same SP chip, they are sometimes specific to one version mpu.
See the following FAQ:
http://newlifegames.com/nlg/index.php?topic=1095.msg5115#msg5115The "sticky notes" (permanent postings) that are located at the top of the IGT S+ discussion area have a lot of good information, check out all the things there:
http://newlifegames.com/nlg/index.php?board=80.0Also, the old NLG archived website has a ton of info about the S+ machines:
http://newlifegames.net/nlg/index.php?board=60.0