...before any takeoff, you must know also the inital climb altitude?
Unfortunatelly, there are still plenty of pilots (however mostly not domestic nowadays), they don't know the altitude of the initial climb after departure.
NEVER depart if you don't know, how far you are allowed to climb! If you're not 100% sure, just ask!
So - how is it on LK** major airports?
It's easy here. The initial altitude for climb departing Ruzyne is fixed and same for all runways and all departures. It is always 5000 feet AMSL.
You may find initial altitude in the descriptions od the SIDs in the charts. However, it is normal than ATC gives any other initial altitude during clearance or taxi, and it will mostly be FL 90, also under to top of TMA Karlovy Vary upper limit.
LKTB & LKMT
On this two airports, you will always be provided with the initial climb limit during clearance or taxi. In most cases expect FL 120.