Different businesses run things differently. Adding to the good advice above, make clear that you are buying a bike but it won't be theirs if you can't test ride it (obviously, be nice and not a jerk when explaining this). Another idea: how about throwing the bike on a trainer? I know it's not the same as the road but you could definitely get a feel for the geometry and make small adjustments along the way.
I've had good experience with this including a shop being OK with me riding a cyclocross bike on a gravel/dirt path to try it out. Another place had me ride a $3500 Trek just for the fun of it when they knew my budget was around $1250 ... they just wanted me to have some fun but it also allowed me to compare the $1200 bike to a really good bike instead of to my old low-end, 2x6 geared steel Bianchi.