You have a couple of options:
1. You could just install ElectronCash (
https://www.electroncash.org/#download).. then open your "old" Electrum wallet using Electron Cash... it should automatically find all your BCH.
2. You can just export ALL your BTC keys from your old wallet and import ALL of them into a BCH wallet (like ElectronCash)... It won't matter if you import "empty" ones...
If you want to just check it out to see what you're going to get... or you really only want to import the ones with coins, you could export the "xpub" from your old Electrum wallet... and then create a new watching wallet in ElectronCash using the xpub... it should find all your addresses that have BCH... you can then find the private keys that match the addresses with BCH.
Personally, I would say that there is no need for you to determine which key(s) had BTC and therefore have BCH on them... just import them all either by using the whole wallet file with ElectronCash, or importing all the exported keys into the BCH wallet of your choice.