Fast ways:
Price
"i" badge on the back (if photos hide this, is 100% rental)
If description goes on about lifetime warranty on batteries = rental
If not mentioned anywhere that's owned batteries = rental
Making sure:
Many 3rd party mot/tax check sites state the full model name (which will have "i" before model name of owned battery)
Calling rci to triple check it is the 100% safe way.
If Considering either, remember that you won't be saving money comparing it with a petrol car if you're paying battery rental, as the battery hire per mile cost is about the same as the difference on cost between fuel and eletricity. (assuming you won't benefit on congestion zone charges etc)
Sure owned battery cars cost 4-5k more, but also remember that you can drive as much as you want at 1-2p per mile without "per mile" fees and that you get all/most of the extra cost on resale (which is also easier and faster to sell privately) unlike the hire one, which the rental battery money is 100% lost and harder to sell privately.
Dont fall for the "battery warranty" sell bs on the hire, specially if still within the manufacturers 8year/100k miles warranty (which covers the owned battery too)
All the best on your search, I gave up 1 year ago as 99% of the zoes in the market were either battery hire, or cost more used than what I could get it as a brand new owned battery + home charger off a dealership.
Ended up buying new, 1 year'ish later (today) I could pretty much sell it for about the same i paid for it.