you go up to whoever is selling it and you say "hey, i ain't buyin' this thing unless i get a chance to ride it first, how can we make that happen?" and then based on whatever that person says in reply you do it and then you ride the bike and make a decision on if you like the bike or not because if you don't like the bike there is no reason to not enjoy a pretty day riding on a bike for free for a little while and if you do like the bike you can still pretend to not like it a whole lot for purposes of negotiating a lower cost for the bike that you enjoyed riding but if you'd rather not pretend then you can say "oh boy what a marvelous bike" and then you can return the bike to the person who is selling the bike and at that point act on the decision you made earlier about if you like the bike or not.