Already a lot of people are trying to conduct this external audit. It's pretty simple. You just need to transfer your Teather (USDT) back to the crypto currency. And then it will be clear if they have enough tokens for repurchase.
No, the way to test it is to convert tether (USDT) to fiat (USD), but even if they give out a small amount of USD does not mean that they have all the reserves, but it would give an indication at least. I am surprised no one on here has commented whether it is possible to do it.