It's hard to measure that directly. Market cap is a decent proxy, albeit inexact. If a coin has a market cap of $1.8 trillion, you know a lot of people are doing a lot of something with it, and it's likely that includes using it to buy and sell stuff to some extent. If it has a market cap of $200 million, then there just can't be many people buying and selling with it, and that means it's pretty likely to be difficult to use that way.