Is there a way to check (or verify) the condition of clutch of a R1200GS without taking the bake apart? Diagnostics, etc.?

No, Diagnostic software cannot measure your clutch wear.

Remove the starter motor. You will be able to see a small section of the clutch and measure the thickness.

Is your clutch slipping?? Is there an oil leak between the engine/trans?

Replacing the clutch on a R1200GS is a bit involved unless you are very mechanically skilled. You basically split the bike in half to remove the transmission and gain access to the clutch. You need a means of supporting the front half of the bike under the engine while you remove the back half of the bike. Disconnect electrical harness from the rear, disconnect brake lines to the rear of the bike. remove throttle bodies etc. takes 2-3 hours for a skilled mechanic to get the clutch out, then 4-6 hours to put all back together again CORRECTLY!

If the clutch is NOT slipping, I would suggest removal of the starter motor on the lower LHS of the engine/ trans. and measurement of the clutch disc before you start the work of disassembly

Normally its the maintenance tray clutch slipping.
Or the maintenance tray ends are worn.
You can remove the left cover and clean the clutch, its on the lower part of the frame, and take your kit out and there are 2 tabs sticking out the ends that were on the last 1/4th inch.

The Printer's Front Panel Displays Printer Error - Report Fault: 07,005.47
Cause:
The Drum Maintenance home sensor actuated, but not at the expected time. Solution: Fixed! Did not fix. TRY FIRST (Solutions without parts): The wiring harness from the clutch is not seated properly on the I/O Left board. Check all wiring connections to the I/O boards and verify. connectivity. TROUBLESHOOTING TIPS AND HINTS: Inspect the maintenance drawer lift pins for wear. See Service Bulletin #445 Rev #1 Replace Drum Maintenance Cam Followers for Inadequate Drum oiling Due to Worn-out Lift Pins. Run diagnostics to verify clutch and sensor operation. Also verify that the drum maintenance shaft is properly assembled. PARTS:Parts are listed in the order of highest probability for repair. NUMBER QTY DESCRIPTION PROBABILITY% MODEL S/N FROM S/N TO S/N 401087100 1 CLUTCH; LARGE,DM,SAFETY CONTROLLED 436041400 1 MAINTENANCE DRAWER W/10K ROLLER & COUNTER/PHASER 860/8200 367051501 2 FOLLOWER,CAM; DM CAM,SAFETY CONTROLLED, QTY 1