It is best to simply play a large number of them in the best stocked stores you can find until you find one you really like. Try all the different types they have, whether it be large body dreadnoughts or smaller body guitars like the 000 size, 000 size, etc. Many people find a smaller body guitar is more comfortable to hold and play...but many also like the large body guitars such as the dreadnought.

I like the sound of rosewood (back & sides) guitars the best generally. In Martins that might be a D-28, HD-28, D-35, 000-28, 00-28, 000-28 Eric Clapton signature model, D-41, etc. Most of these guitars have a spruce top. Cedar top guitars can also sound very good, but those are rare with Martins.

Don't think it will automatically be a great guitar just because it's a Martin! Not necessarily. I've tried out Martins that were mediocre, frankly, although they were still well-built instruments....but they just didn't have a really good sound to them. Like with any top end brand of guitar, some are definitely better than others, and you need to play a lot of them to make comparisons.

When you find one that you just like one a lot for some reason, better than you like the other guitars...that's the one for you.

I go by sound, feel, and general appearance of the instrument. (as well as price level, of course) If all that pleases me, then I'm interested in buying it...whether or not it's a Martin.