Well, seeing that you are in Turkey, I'm not sure how much advice I could give you.
In the US, if you buy from a reputable dealer, you should be all right, but I don't know if there are direct distributors of Pokemon cards in Turkey and even if there were, older boxes like Base Set 2 are all bought on the secondary market.
Generally, a store selling cards knows where they got their product from.
A real telling point is if they are selling the packs at a very low price.
Generally, in the US, discount stores that are selling the counterfeit product price it very low.
They sell packs that should cost $3-$5 for a mere $2.
If you're seeing product being sold at retail stores for below wholesale prices you know that something is up. They must have gotten the product very cheap and that is only going to happen if it is fake.