You are the customer, as well as medical professionals they also run a business - one that you choose to use. Ask if there's any benefit in doing tests. Say you're worried and you would be greatly reassured by tests. They may give a good explanation as to why giving it a few weeks with Baytril before going down that route would be better and you will come away feeling the treatment is appropriate - or you could just say you are willing to pay for tests whether they are strictly necessary or not.
I'm a pain at my local vets now when I use them. I do actually feel I know more about rabbit-specific issues than they do
Thankfully they have been very good at listening and haven't (yet) told me to shut up. They do know that I know my stuff (stuff that I venture an opinion on, at any rate) and have learnt much of it off the most renowned rabbit vet, so they let me spout!
I'm also quite willing to raise anything with FHB and she is happy to listen and discuss. She knows I gain information from 'knowledgeable people on the internet' and she has respect for the value of this. She will be happy to tell me why she wants to follow a particular course of action but wouldn't be offended by me asking 'why' and 'what if'.
No vet should be offended if you ask in an appropriate way. Don't be afraid to get the service you want - they are only in business because of your custom.