The metrics always look good before a recession and it's usually an 'unknown unknown' that sparks it. Aside from the big short guys, noone thought 2008 would happen.

The periodicity of recessions are typically 6-8 years. We are at 8 years now. However if you want examples of specific warning signs:

The markets have been consistently hitting ATHs over the last 6 months. There have been numerous warnings that they are over-valued, particularly tech. There have been warnings over the dangers of US student debt. There's the danger of political turmoil because a donkey was elected into the White House. Chinese overall and hidden debt has been rising beyond sustainable levels. The housing markets in most major global cities (San-Fran, NY, Toronto, Sydney, London, Paris to name a few) have basically continued their bubble growth bar a 2 year interruption after 2008, largely funded by foreign speculation (which takes us back to the Chinese debt). There have been warnings that the long period of quantitative easing are leading us into a period of stagflation not seen since the 70s. If that happens again we may see currency devaluations and double digit interest rates. I'm sure you can imagine what that would do to the housing markets and the Chinese debt i keep mentioning.

As i said, these are my speculations and probably when a recession occurs it will be caused by something that noone thought of. But the risks are there.

I sold my bonds and equities last month. It's been a great run and i exited the market after a consistent run of ATHs. I can afford to lose a year or 2 of growth. I will buy back in once the recession kicks off. I'm currently planning to HODL my cryptos through the storm since there isn't any precedent for how they will behave in a global recession. My feeling is that they will drop as investors seek liquidity and safe havens. If they do i will buy more so i have kept money aside to do so.

ALL SPECULATION, DO YOUR OWN RESEARCH