-As has been said, travel far and wide and play as many as possible, comparing them side by side with what you currently have.
-If possible have repertoire that covers a wide spectrum, but try to play the same short passage on each guitar.
-Bring a clip on tuner with you for quick and easy tuning. It's best to judge each guitar at standard concert pitch. Guitars that are out of tune could also cause you to wrongfully judge the sound of a guitar
-I would prefer to buy used as you get a lot more bang for your buck. I also would buy after trying in person rather than buying blindly through the internet
-Judge each guitar without bias as much as humanly possible. Don't be swayed by price, luthier, woods used, construction methods (traditional, lattice, DT, etc.)... If you have clear judgment, you will sometimes be surprised how less expensive guitars outperform more expensive ones
-In the end, sound and playability is what matters most. Don't be swayed by huge volume and projection. Volume may impress you at first, but it is beauty of tone that will keep you falling in love with your guitar.
-Be patient. It is sometimes tempting to get a guitar TODAY, but waiting a bit longer, testing more guitars, waiting for good used guitars to show up on the market will be well worth the wait and will allow you to maximize your budget.
One last thing. James Lister's #100 guitar is still for sale on the forum at an excellent and discounted price for delcamp members. I would seriously consider making the drive to try it out as it is right within your price range and looks very promising. Here's a clip of Marcin playing it and it has stunning tone: https://www.youtube.com/watch?v=SoF4c1RBDlo
"Success grants its rewards to a few, but is the dream of the multitudes.
Excellence is available to all, but is accepted only by a few." - Christopher Parkening