You don't have to actually sell. If you don't have a product yet, just build a mock payment page and check how many people get to the end - and never charge them.
An idea I just thought of on that front - instead of the usual bland "we cannot fulfil the order at this time" end to a dry-test, send people to a 404-alike page with a contact email. You can track conversions by watching the clickthroughs, but as an added bonus, if people subsequently email going "your site's broken! Where can I buy?" you know they really want your product
