There's some nice chunky research being done by a group at King's using an app where people plug in their symptoms, then they see who gets COVID and who dies etc. Here's a recent summary of their findings:

https://www.kcl.ac.uk/news/six-distinct-types-of-covid-19-id...

Headaches and loss of smell are the most common warning signs of COVID, but they don't predict whether it will be a serious case or not.