Go to the pool store (or Home Depot, Lowe's, etc if they sell them) and get a new pressure gauge for the filter, and a roll of Teflon tape. The gauge is only $10 or so, and it only takes a few seconds to replace. Unscrew the old one, wrap the tape around the threads for the new one, and screw it back in....that's it! Backwash the filter, rinse it, then turn it back on, and note your "clean" pressure reading. Then when that pressure increases by 6-8 psi over the "clean" pressure, you know it's time to backwash again.

You can open the sand filter up (if you want to post pics via photobucket, flickr, etc, one of the equipment guys on the forum can help you identify the equipment and tell you where to open it) and look at the sand--if it's not gummy, then there's really no reason to change it. Sand almost never has to be changed, unless it's gummed up from a Baq conversion or other chemical. My other 3 co-mods are each over 10 years on the sand in their filters, with no problems. I would be, too, if I hadn't had a filter problem a few years ago and switched to zeo sand when I filled it back up, just to see if it was any better than sand (it's not, it's just more expensive ). You might, however, need to top it off--most of them should be 2/3 full with sand, I think--does the owner still have the manual? If you DO decide to change it, you can get filter sand at the pool store. Sandblasting sand, sandbox sand, etc. is not the same stuff--it's a different coarseness. It's not a hard job, it's just boring and labor-intensive....it requires scooping all the old sand out and disposing of it, and then scooping the new sand back in, being careful not to damage the laterals inside the filter. Personally, I would just replace the gauge , if everything appears to be working okay, and then deal with the filter later if you need to. I'm a fan of "if it ain't broke, don't fix it!".

How does your water look?