Execute DomainNamesAvailability_API request with OperationType=90 (Transfer) for tld that is checked by OPENSRS registrar. See the following message in API result (for any domain):
<string>Domain is already registered. </string>
Issue is specific for OPENSRS registrar only.
Requests actually returns correct DomainStatus: 1 - unavailable domain name or 2 - available domain name, so you can ignore the "Domain is already registered" message that comes along with it.
If you get DomainStatus=1 for domain that is actually Registered and logically should be available for transfer, it means domain is locked on Registrar side and indeed can not be transfered at the moment.