This depends on how the Payments API has been used:
Remitter account details specified
If the remitter_name, remitter_account_number and remitter_sort_code parameters are specified when creating a new payment, then these will also be visible in resource representations provided in the GET response.
Remitter account details not specified
If the remitter_name, remitter_account_number and remitter_sort_code parameters are not specified when creating a new payment, then these will not be visible when querying details of the payment using the GET method.
In this case, although the user will have selected the account they are using to pay from as part of the payment consent journey implemented by their bank, these details are not provided by the bank to TrueLayer when the bank executes the payment, therefore they will not be available via the Payments API.
To determine the remitter account details, it will be necessary to look at the deposit transaction in the beneficiary bank account statement (in almost all cases, this is the bank account specified using the beneficiary_account_number and beneficiary_sort_code parameters). This is possible either by separately using the TrueLayer Data API with that bank account or by logging in to the online banking portal for that bank and manually viewing the transaction.