I am deposing a client's spouse about bitcoins they have mined since 2013. What relevant questions should I ask to ascertain number of coins mined and transactions concerning those coins, such as number of coins still held and location of coins?
To 'own bitcoin' is to have knowledge of the private keys that allow some unspent transaction output (UTXO) to be spent (though in some cases, knowledge of a key may be delegated to a custodial service). So understand that the 'bitcoins' themselves are not what a user must keep safe, rather it is the keys that allow those coins to be spent.
With this in mind, consider that there are many ways to store bitcoin keys. A savvy user may be able to obfuscate their keys entirely, and there is not necessarily any way to prove that a person has current knowledge of a key.
"How much bitcoin have you mined since 2013" is the obvious question to ask. Consider that some amount of the bitcoin mined may have been sold in order to finance the mining operation, may have been traded into other cryptocurrencies or assets, may have been lost due to user error, etc.
Otherwise, I agree with @RaghavSood's comment, especially considering the legal implications of this question, it is difficult to answer comprehensively in a SE posting. This seems like a situation in which a paid consultant may be appropriate.