I was given around 150 grams of gold (none of them are jewellery) as a gift for my wedding. Therefore, I don't have any receipt. I wonder if this is problem. How can they check the real value of it in customs?
If the gold is in bars or similar with a purity of at least 995/1000 or in certain kinds of gold coins it is investment gold and treated as currency. If the combined value of the gold and any cash and other monetary instruments is € 10 000 or more, a declaration is required, but no taxes or duties are due. The metal value of 150 g of pure gold is about € 6 600.
Otherwise, the gold is subject to VAT based on its retail value.
The real value depends on weight, purity, and current market rates. Weight is easy to figure out, they will have scales. Purity is a bit harder, but usually engraved on the bullion. Market rates are easiest of all, they're published constantly.
Declare the gold, and take the red lane just in case on arrival. You might be asked to pay import duties that way, but that should be all.