Inspired by this feature request by Shadow Wizard, I wrote a simple userscript (direct install link) which checks most of the reasons why you can't start a bounty, and displays that information on the same place where you'd normally find the 'start a bounty' link. It checks:
- if you're on a Meta site (those don't have bounties, except for Meta Stack Exchange)
- if there's an open bounty
- if the question is closed, deleted or locked
- if the question is at least two days old
- if the user has at least 75 reputation (where you get the bounty privilege)
- if the user has posted an answer, which raises the bar to at least 100 reputation
- the last bounty by the user, the new one has to be at least twice that amount
- if the user has three active bounties already