If I have a transceiver using 868MHz and connect that to e.g. a Raspberry Pi and start reading from it, will I be able to see all traffic on that frequency happening nearby? So if I have a sensor that is sending some data periodically on that frequency I will see it somehow?
Generally no - you need to match the exact frequency AND modulation (OOK? FSK? QAM?) in order to make sense of the signal.
In practice? Quite often these things use standard modules and you get lucky with it Just Working.
The fully general solution is Software Defined Radio. That genuinely does let you pick up everything in a band and try to make sense of it in software.
Typically these are wrapped with protocols that allow you to get your information despite other broadcasts on the frequency. But technically yes, you can see the raw data on the frequency. Sometimes multiple devices on the same frequency can interfere with one another. Which is why it is important to have a protocol so that devices can work together rather than step on each others toes, so to speak.
As far as the sensor goes, your Raspberry Pi will have to understand what the sensor is broadcasting. The datasheets should provide more information on this.