Europe and Japan are still very problematic however.
The simple fact that people, businesses and banks hoard cash and government bonds instead of doing sufficient investment to keep people employed is strong evidence to me that the returns on government paper is above market. Excess reserves are high into these parts of the world.
A more widely recognized sign is that they keep undershooting their inflation target.
For arguments from real economists, look around these parts https://www.davidbeckworth.com/popular, https://worthwhile.typepad.com/worthwhile_canadian_initi/nic... or https://faculty.insead.edu/fatas/
One thing you can't rely on is the fact that central bank rates are zero in the problematic places. Zero is a high, above market rate when the equilibrium is negative. As I tried to emphasize in my medium post, negative rates can be quite normal and natural. If marginal private assets don't have a positive real rate on a risk adjusted, liquidity adjusted basis, the government shouldn't provide an artificial substitute. If they do, it puts a gridlock in large parts of the economy.