Know your components, sometimes even something like a saddle has enough value to justify the price. Looking through that lens, the rest of the bike is "free". If I don't know the bike off the top of my head (this is true like 95% of the time), I make a decision to buy based on components. Learn at least a rough framework of the hierarchies of shimano and suntour, and get a sense of what makes "nice" components nice. Things like finish on the parts, alloy vs steel parts, even feel and quality of an unknown part can offer some idea of the value of the bike.

Honestly I've never put much into the brand or the specifics of a given bike, craigslist scouring dictates a quick hand that works largely on educated guesses. But by knowing what to look for, and picking up on cues to value, I can resolve all the specifics at home, once I've bought the bike.

FWIW, I got each of these bikes in the last year for less than your $250 target.





... and if you doubled your budget, every bike in the gallery in my sig would comply.