The U-joint is out of sight in the hollow swing arm and requires no maintenance for many tens of thousands of miles, well over 100,000 miles. A few have failed at less than that but it's rare to have a problem at less than 100 or even 200,000 miles. There are two sistered bearings inside the splined hub, the so-called driven flange, that sometimes fail at less than 50,000 miles. These are not the wheel bearings - these two small bearings only support the driven flange loads. A large circlip retains the splined hub in the rear wheel. With the splined hub removed you can check the condition of these two bearings by removing a collar spacer that sits pressed inside these two bearings. Also at this time you can check the RH wheel bearing. Otherwise the RH wheel bearing is hidden under the driven flange and can't be checked. The two wheel bearings in the wheel are very long lived - 100,000 miles or more.
A stock suspended ST1300 has a lot of sag under normal rider weight. It's typical to have 45 to 50 mm of rider sag or up to 46% of travel. Maybe yours has had the springs replaced with stiffer aftermarket ones if you have little sag. Sag should be in the area of 36 mm.
The driven spline is installed as part of the wheel so when the wheel is removed the spline is accessible for cleaning and regreasing with Moly 60 paste. The service interval is commonly the length of the rear tire life but on a new used bike it's worth pulling the wheel to check for moly grease on the splines.
This tech article will be helpful to ascertain the position and condition of the two small flange bearings:
https://www.st-owners.com/forums/showthread.php?55014-ST1300-Rear-Flange-Bearing-Replacement
