> What would you recommend doing if I wanted to see which are valuable?

Attempt to sell them. That's the only way you can establish the value of anything.

One easy way to get an idea is by looking at recent transactions for similar domains (do not look at the asking prices, they are typically not realistic).

> And secondly how do I go about finding buyers?

There are many ways to attract buyers, advertising, auctions, brokers, messages on the domain itself, a blog post. Whatever you feel is required.