Depends if it is registered or unregistered title, first off. Assuming it is registered (chances are, it is) then there should be an easement recorded on the title.
If not, then either a) there is no right of way or b) there may be a right of way by prescription if the seller can provide you with 20 years + evidence (such as statutory declarations) of use.
You can check it the title for the property in a few minutes at the LandRegistry website for a small fee, very easily. BUT, this is the biggest purchase of your life and you need to be sure you've got a decent solicitor acting for you - this should be bread and butter to them so if they are not on to it, consider whether you really want them to act for you. Maybe discuss it with a senior partner first, and if you are still not happy, the complaints partner, to ratchet up the pressure.
One other point, if there is no ROW (when it was assumed that one existed), it is likely to affect the valuation so - assuming you still want the house - you may need to get the valuation checked/ inform the mortgage co/ decide whether to put in a lower offer. Again, your solicitors should be dealing with all of this and keeping you informed. Unfortunately there are some rogues out there, especially at the cheaper end of the market, so don't be fobbed off.