all the responses are people taking the word "wallet" too literal as software..

the OP is talking about if some hacker or ex girlfriend obtained the seed phrase/wallet.dat file thus has possession of the private key..

in which case if they have not already moved the funds out of the keys to their address thus stealing funds away from you. the only option is for you to quickly move the funds to a fresh seedphrase/wallet that uses different keypairs which the other person does not posess

if the funds are gone. and you are trying to take this person to court and you are asking for proof that you owned/created the seed phrase and not them. to claim original ownership, meaning proving they stole your funds
well thats alot harder to prove

unless:
a. the phrases represent words that can clearly be tied to things in your life and not the hackers
EG all 12 words have a meaning towards your hobbies EG your a musician and all words are musical related. and/or have relation to your families hobbies/interests

b. where the stolen funds came from before being sent/deposited to your keys. EG buying coin on an exchange and withdrawn straight to your wallet whereby the exchange has KYC evidence that you bought the coin(though the thief can just claim you donated coins straight to them when you withdrew from the exchange) but would require the thief showing equal proof that the funds were intended for them to spend EG needs conversation logs of you saying you will buy coin and send it to them direct from the exchange