One of my strategies for determining fake wallet.dat files is checking bitinfocharts.com (Explorer -> Bitcoin explorer).
If you paste the address you're looking for, you may see that it's part of a multi-address wallet (green link next to "Balance").

For example, using this wallet that freude2021 talked about:
1LHjrPBeEYQv1f219VQcQ71EBpwvp3hdD5 - 8.9 BTC

Actual balance: 39.9 BTC
Addresses that should be in the wallet.dat:

Conclusion: fake wallet.dat (unless 1LHjrPBeEYQv1f219VQcQ71EBpwvp3hdD5  is the first address, and this is the first backup of the file - unlikely).

The other wallets, 15.9 and 75 cannot be checked, as there are not useful transactions in those wallets.
36.6, I'm not sure which one this is (not present in allprivatekeys)