[bob778]

It’s the symbol to the left of the URL > Show Certificate. They even make it available on iOS Safari (Page Info > Connection Security Details), but if it’s expired, you’ll know by the big red warning page.

[jwr]

I don't have that :-) — what I see when I click the thingamajig on the left side of the URL is a menu with "Hide Distracting Items", "Zoom", "Find" and "Website settings".

[loloquwowndueo]

For me it’s hidden behind a kebab menu in that thingamajig you described.