How do I know how much a foreign coin is worth?

As an avid collector of foreign coins, I often find myself wondering about the true value of the pieces in my collection. But how do I accurately determine the worth of these coins? Should I rely solely on online marketplaces or auction sites? Are there any reliable resources that can provide me with a comprehensive analysis of the coin's rarity, condition, and potential value? Moreover, how do I account for factors like demand and supply fluctuations in the market that might affect the coin's price? I'm eager to understand the best practices for determining the true value of foreign coins to ensure I'm making informed decisions when it comes to buying, selling, or trading my collection.