You could certainly ask the expert about that. In my view, it's in the interests of both you and the expert for that to happen.
The expert isn't working for free. Perhaps the expert will only look at the material once you have confirmed the call time. That gives you (as a client) more of a commitment to this expert. But you don't get any feedback until you get a paid call.
As someone who offers advice over Clarity re positioning and pricing, it is certainly in my interest to give the very best advice I can. An important component of that is to do what research I can do ahead of time, so being able to have at least a quick look at a website, a slide deck or a short PDF, before the call makes for a better quality call.
Better calls = better reviews, which is attractive for other potential clients.
For example, supposing a creative design agency contacts me and says: "we need help with positioning ourselves in the market. Our clients are comparing us with Upwork." If I have some background on this particular design agency, by assessing [for example] that they impress visitors with their amazing designs, but that rarely leads to conversions, then the call will be more efficient. Shorter calls are likely to build trust for longer calls and perhaps other project work.
I respect the decision of experts who say: "no, we'll look at this once the meter is running." That is also a good sign, that they are respecting their own time, which increases their perceived value. But reviewing on the spot may require some quick thinking, and being prepared ahead of time makes for a better result for the client, in my view.