Today I joined 2 races on CTRA 1b, but at the end I got a time penalty and was disqualified. I understood from others that this was because I did not make an obligatory pit stop.
Is it, at this moment, in all races obliged to make a pit stop ?
If not, how do I know ?

Normally no pits on bronze servers. Its probably endurance night, longer races on average and pitstop required. It should show on race start by servermanager and lfs itself and also by looking at F12.

it says by F12, and a message appears "Compulsory Pitstop: On or Off"
and if u look that other drivers are pitting , just pit

I do believe it's Endurance Day on there, as Spikey said. Pitstops are compulsory on endurance days. At the beginning of a race, in yellow in the middle of the screen (IIRC) it says "Pitstop Required: yes" when a pit is required, and of course as chanoman says, you can see whether it's required (and if you've already made yours) from the F12 menu

Thanks for the replies.
Must have overlooked it in my eagerness to join.

It surprised me that it is possible to earn lots of points even if you are disqualified because you did not pit. If I remember correctly, yesterday,i forgot to pit and was therefore disqualified and i still got more that 20 points.