How does one tell the web site for a refundable fare?
Not possible to the best of my knowledge. But any ticket booked through the standard web reservation page (vs. the weekly specials) is, in practice, refundable before the tickets are printed.
Actually...if you request one of the standing discounts—AAA, NARP, Senior, Military, etc.—you might get a refundable fare almost every time. These discounts are typically not combinable with special, non-refundable fares. Not a 100% sure bet, but this is the closest you might get to ensuring a refundable fare without reserving over the phone.
Keep in mind there's still a refund fee for printed tickets.