The very best thing you can do is purchase (and read!) a copy of "The Piano Book" by Larry Fine. If you are buying a relatively new piano, you should also get the 2003 supplement.

The book will give you enough of an education that you can weed out most of them on your own, then you can hire a tech to look a only the really great ones.

Play all the pianos you can get your hands on. If you've decided on a model, so much the better. Play a bunch of them because they all have different personalities, especially Steinways. Oh, and if you're considering a Steinway, you owe yourself a visit to Steinway hall in New York City. I was able to play about a dozen Model B's (7 foot grand) including I think 8 of them at Steinway Hall before finally finding a truly remarkable one on Long Island.

Mine is a 1973 model with the Teflon action that is supposedly terrible but has never caused a problem for me. The tone is far superior to any of the new ones I played.

Edit: try to find yourself a good tech locally. They often know about good pianos for sale.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
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