I've heard the same thing about the UN-54s having a hollow axle. I'm not sure what the difference is between the 54 and the 7x now. Maybe bearing quality?

As for the OP, the length of the bb spindle is usually stamped on the drive side near the bearing, or on either a sticker or stamp on the bottom bracket shell if you pull it off the bike. The biggest issue with bb length for a geared set-up is chainring/chainstay clearance. If you have plenty now, the bb won't make a huge difference. If the chainring comes close to hitting the chainstay, you'll probably want the longer spindle just in case.