Sent BTC to BCH wallet
Your method is correct: you'll need to use the mnemonic seed phrase that generates the bch wallet, to generate a btc wallet. If you do that, you should see your btc balance.

To see if your btc are at the address you sent them to, just use a block explorer website such as blockchain.info. When you use the mnemonic to generate a btc wallet, it should show you the sae addresses as the bch wallet (as long as the software uses the same derivation scheme. Most software does).