Any manufacturer"s name visible on the valve... if so, a "net search might turn up how the ports should be labeled.
The smaller hose is "pressure", and the larger hose "return" 90+ % of the time, UNLESS someone has been able to muck things up and figured out a way to reverse them!
ONE result of hooking them up backwards will be lots of pressure on the seals at the end of each spool.
Since this is a closed-center system, it does not have a relief valve in the block you are adding.
If it were an open-center system that would be a BIG problem, "cuz hooking it up backwards can break the pump, the valve or blow lines because of the relief valve not being able to function!