Right now it's kind of persistently slightly irrational. A quick-and-dirty measure of its sanity is the P/E ratio: how much money are the publicly listed companies actually making? Right now, the number is around 22[1], meaning a dollar invested in the market takes 22 years to pay itself back purely in terms of corporate profits.
That's a return of about 3%. Numbers over 20 are generally considered a sign that the market is overheated. They peaked at around 45 and 65 right before the 90s and 2000s crashes. So there is revenue there, and not entirely out of line with stock prices, but not entirely line with them either.
None of that takes coronavirus into account; that's current price with last quarter's earnings. But if you assume that coronavirus is only temporary (it may not be, but let's be optimistic for a second) it means that long term the numbers are calling for a crash... eventually. One of these days. Longer than you can remain solvent while shorting it.
Probably.
In other words, I've got no idea what's going on. The markets have been weirdly stable (in price-to-earnings terms) for the past decade, at a number that's too optimistic, before coronavirus. Surely the inevitable at-least-short-term loss of earnings should have corrected that, but it hasn't. So either people are thinking very long term, or they're just nuts.
Eventually it will because businesses will never recover without consumers recovering. That said I don’t think it’s looming...again congress gave over $4T to the FED to stabilize the market. To put that runway in perspective consider how in 2008 we were on the brink of a total financial collapse, yet with only $1.6T used to stabilize the banks the financial collapse was turned into the longest bull market in history (over a decade) and record high markets. We are talking 2.5x that bailout this time around.
Once Q2 earnings hit next month you'll see a massive market correction.