I've done OAuthV2 from user for adwords management, now I want to manage client's account and for that I want only those accounts on which user has admin access. Can you please help me how can i get those accounts OR at least how can i check whether client has admin access or not on given account?
As far as I know, the Adwords API offers no possibility to check the access level of the user whose OAuth2 credentials you are using.
To see whether or not you have write access, you can just try to create a new label or similar and check whether the call succeeded.
Maybe you could do something similar and attempt to add a new account to an MCC using the ManagedCustomerService, but I'm not sure if that actually requires admin permissions.