It’s written on your pass, and mentioned on the QR-code page with mobile passes.
Ticket inspectors check this, when they look at your ticket and check if you don’t travel in 1st with a second class pass.
With a 1st class pass, it’s not a problem to travel in 2nd. But it is in the other way around.
When reserving 1st class seats, you need to indicate yourself in which class you travel. The websites don’t know this. But obviously this also gets checked at inspections. Seat reservations in 1st cost a bit more than seat reservations in 2nd class, like ordinary tickets in 1st also cost more than tickets in 2nd.
The site selling the seat reservation with sell you an any reservation you like - it doesn’t ask you for your ticket. You might have no ticket at all.
But it’s just a reservation, not a ticket. The conductor on the train will want to see your ticket or pass.
If you travel in first class with a second class pass, you may well be viewed as travelling without a ticket.