Mark Tandy wrote:

How can we tell what edition we have

Well, you seem to know the year. Can you find the publisher? That
information is usually given on what's called the colophon page, which
is generally at the front of the book, often on the reverse side of the
title page. However, 18th century books often give all those details on
the title page itself.

what is 'the OP's edition'?

That's the one you've got; you are the OP (original poster) in this thread.

Many books of that period have a complicated printing history (there are
often separate editions for Ireland and Scotland, for example, and books
may be reprinted by separate publishers over a period of years), so it
may not always be possible to say that your copy is the 27th edition,
for example, or the 35th. The main thing is to find out when the first
edition was published. You can usually get that from ABE or AddaLL
listings or from library catalogues. Here is a good metacatalogue, that
gives library listings for a large number of libraries:

http://www.vialibri.net/library_search.php

Once you've established that the book in question is or isn't a first
edition you can start comparing with other books of the same type. An
18th century reprint edition of a 17th century play will compare with
other similar reprint editions. If you can find one of comparable type
in comparable condition to yours listed on ABE or AddaLL then at least
you know what one dealer is asking for something that is in the same
ballpark as what you have in your hands.

That isn't always 100% helpful, though; another dealer might ask a
different price and, anyway, these are *asking* prices; they might not
actually sell at that price. Also, these are dealers' prices and you are
not a dealer. Your best chance of realising anything approaching
dealers' prices is probably eBay. Take a look there, too, and see what
kinds of prices comparable works are fetching.

John