There is an excellent, reliable, time-tested way to find out all about the action (and other important elements) of a guitar before you buy it: you play the guitar.
Yes that does present a challenge when buying online. When shopping online you have three options:
- Make sure you buy from someplace that has a great return policy and you have the interest and ability to use it. E.g., make sure you have a favorite place to go to ship unusual packages so you can send back any guitars that don't have the feel you are looking for.
- Instead, you could look to only buy guitars that have a decent resale value. If you can't send it back, you can send it forward. You might take a loss on those kinds of deals, but even when shopping in person there are costs to even consider buying something. When I bought my most recent amp, I drove all over Southern California trying out amps, and spent a lot on gas and meals.
- Finally, you could accept whatever action you get, keep the guitar, and have a tech or yourself make all possible adjustments to make it as playable as possible.
Modern manufacturing makes the third option above more viable than you might think. Computer controlled milling has greatly improved the geometry of low end guitars, but only from companies that have made those investments, and only on guitars made in the last five or ten years. That means the cheapest possible Fender or Taylor acoustic could have a better action than a higher priced guitar from a smaller brand.
Speaking of Taylor, they actually invented a system where their acoustics have an adjustable neck angle via hidden screws. Their guitars are much more predictable in the area of action. Buying a low end Taylor from Sweetwater (a site that is great on returns) is an option that might work well for you.