You have to inform yourself about the laws in Croatia, here in Germany it is not legal outside your own property. Items of archeological relevance have to be handed over to the authorities anyway, we cannot keep or sell them. And anything else is only scrap metal or stuff from WW2, which could even be dangerous.
You just do it, as long as you're a citizen. If not the soldiers or rent a cops will probably run you out. Apply for a job & go over, you'll be stopped & warned, but as long as you know that there's so many mines & unexploded ordnance you'll can search till your is content