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.

If people are approaching you to buy them that is one indicator of value.

In general domains are a numbers game. If you own a large quantity of decent names you might sell 1% of them a year. Whether you get a good number totally depends on how you do the negotiation that's a huge factor.

In general it's pretty difficult to outbound sell domains especially if you don't know the business to be able to size up and potential buyers.