Search the cards on eBay: for example, say you have a Japanese Nidorina 40/64. Look it up on eBay, then filter by "Sold/Completed Items". Doing this shows one of the very few Japanese Nidorina cards sold recently went for about $2 USD, so you can assume that card is valued at around $1 - $2 USD.
Sites like trollandtoad, while helpful, cannot give as accurate a measurement as eBay Sold Items, as that shows you what prices cards *actually* sell for, not what sites estimate they *should* sell for. Only use trollandtoad/whatever other TCG sites there are if you can't find the card you're searching for listed on eBay.