99% of the time it’s obvious, especially with modern cars that read the signs plus apps like google maps (though sometimes at least one of those is wrong). You can also look around and see how fast everybody is going, eg. when you enter the highway in an unknown area, and not sure about the speed limit for that section. If I’m uncertain, I just drive normally until I see a sign that comfirms the speed limit.
I am regularly going around 200 km/h, and the only ticket I got was going 99 at an 80 sign next to a closer and renovated lane. It was around midnight, I was already slowing down, but didn’t want to slam on the brakes harder.