https://www.usertesting.com - you can upload screenshots / prototypes / the actual webpages (if they exist) and have users go through a series of questions. You can also set it to scramble the order to see how that (if at all) changes preferences.
As other folks called out, this can give you a sense of customer comprehension / stated preferences & reasons why, but may not give you the level of confidence a true A/B test would (launching both iterations & splitting traffic amongst & measuring conversion difference)
As other folks called out, this can give you a sense of customer comprehension / stated preferences & reasons why, but may not give you the level of confidence a true A/B test would (launching both iterations & splitting traffic amongst & measuring conversion difference)