Had that recently with daughter's bed.
I normally prefer to lie onto the mattress in order to know if it'll be good or bad.
Online at Amazon, it's been an odd experience as it was reading one description after another. Plus reading positive and negative reviews.
Thankfully, it was a great choice and we both feel as if we're on a cloud.
My own king-size I'd got from British Heart Foundation. Bargain galore at 1/3 the price. The reviews I'd found online were bad.
Frankly, I disagree after a year of such comfortable sleeping surfaces. Less pain and ease of getting up every morning.
Two sides of the coins which however strange and odd were good experiences.