The most common symptoms of a loose ribbon are low output and decreased low-frequency response. With your mic open, it’s easy to see if the ribbon sags. But with sealed, un-modded mics, the protective mesh hides the ribbon. So If you suspect your ribbon needs tightening, but don’t want to disassemble the mic to check, Michael recommends a quick and dirty test: Connect the microphone to a mixer, and listen through headphones while rocking the mic back and forth. If the ribbon is slack, you’ll hear a clanging sound when the ribbon hits the magnet.
One or both of the ribbon clamps must be loosened and removed. Then the ribbon pried gently away from the lower clamp surface. Occasionally the ribbon is fused to the clamp surface and cannot be moved without tearing. In this case the last resort is to hope the other end of the ribbon can be freed from the clamp and pulled taught.
You might also need the previously-noted page about replacing the ribbon element in a ribbon microphone.
(And the easiest way, especially if you’re all thumbs: Have OktavaMod repair the mic for you.)