CMOS inverter: circular reasoning to understand it?
To identify where is the source and the drain of a transistor, I must find which one has the highest voltage.


But as I do not know \$V_{out}\$ how can I find them ? \$V_{out}\$ is what I try to find so I am not supposed to know it?

You know that the output voltage will never be greater than \$V_{DD}\$ or less than \$V_{SS}\$.
Therefore, for the PMOS device on the high side, the source is always the terminal connected to \$V_{DD}\$ and for the NMOS device on the low side, the source is the terminal connected to \$V_{SS}\$.
As others have pointed out in comments, if you're building this from three-terminal discrete MOSFETs you must properly connect the terminals designated by the manufacturer as the source and drain correctly or the body diode will conduct, and the FET will not be able to block current. If you're building the device within an IC, then you'll be able to connect the bulk terminal to the correct voltage rail, and the device will behave symmetrically, provided you don't try to drive either source or drain terminal beyond the rails.