How to tell if Raidho ribbon tweeter is damaged?

Why not reach out to Raidho?  Unless someone here has extensive knowledge of their parts and pricing, go to the manufacturer. 

If it's not buzzing, it's most likely good.

If it sounds okay to you, then it's probably fine.

Most ribbons you can see the blown ribbon. They normally just quit working. A planar sometimes will make noise when they delaminate.

If you can’t hear it, pull it, and check resistance with an DMM. If it check out, go BACK check the wires, check the XO..

Happy hunting..

Thanks so much for your advice.

According to advice from Raidho, the tweeter can be retightened by heat gun (like the video shared on Raal ribbon tweeter).

Please share your advice if ribbon tweeter will sagged after it has been used for some years, especially in South East Asia where the weather is humid and hot? How long would it last usually?

"retightened by heat gun"  Yikes!  So it's just like tightening the mylar skin on RC balsa airplanes then - right?  The benefit of RC airplanes, when you screw up, you just cut another piece and heat tack it on; on a $10K speaker, eeeeh, embarking on a DIY using a heat gun sounds like...  an air ground crewman seeing a cladding defect on a B-1 bomber, then taking the bubble gum out one's mouth and just covering it up.  Yes, just McGyver it.  Sounds good to me...  Hah!

If contemplating use a hairdryer it will provide 
as hot as required. Perhaps your nomenclature/language  
was actually a hair dryer as a "heat gun" will be much 
hotter and not recommended at all. Danger!

In fact, the advice is to use "heat gun" like that in the video for re-tightening of RAAL ribbon tweeter: https://www.youtube.com/watch?v=bvylZF9eJOA. Yet, the advice also said that a hair-dryer might work.
I also feel the "danger" of doing this to a >10k speaker. 
Yet, no one can tell me the cost of replacement and if it is worthwhile to do so.

Hello cclee2022.  Did something happen and the sound suddenly change? If so, it could be the speaker or the crossover; perhaps even the amp. Try switching the the speaker wires, left to right, etc. If the sound problem moves, the speakers are OK. Further testing will require a known good capacitor (not an electrolytic), 5-10 mfd, 250 volt or so. Gain access to the tweeter's input wires. Disconnect one of them. Connect the capacitor to the disconnected terminal. The other end to the capacitor goes to the + side of the cable feeding the speaker. Disconnect it from the cabinet of the speaker. Connect the - speaker cable wire terminal to the other tweeter terminal, the one you didn't disconnect.  Play some music at a normal level. Pull one wire off the other (working properly) speaker so it doesn't play while testing. (You may be surprised how little sound comes from the tweeter.) Does what you hear sound clean and clear? If so the speaker is OK and the problem is in the crossover. Get help in troubleshooting the crossover. You will probably have to pull it out of the speaker cabinet. Happy testing!

OP I watched the video. It shows the right tension. You blow on the ribbon at low speed on COOL and look at the flex. It uses shrink material at either end.

If I was going to retention I would look at the amount one end or the other was already shrunk, heat the one with the least amount first. It won't take much, that's for sure.

Like any shrink material, it get brittle the more you heat it and it cools..

Regards

Heat gun... Danger, danger Wil Robinson.