This does not answer the question directly but I do not think the right question is being asked here. It appears to me the asker wants a program which will act different presumably if it has certain permissions or not, however I would argue checking for sudo is not the way to do that. Firstly many systems may not implement a "sudo", it is by no means required on Linux or many Unixes.
For example a user may be logged in as root already, making the sudo nonsensical or perhaps the system has non root users who still have capabilities to perform the administrative task that the program may wish to do. Finally perhaps the system has no root or sudo at all and instead uses a mandatory access control system with different capabilities and no catch all superuser to sudo into. Or the user could be sudoed, but into an account that has -less- permissions than their own account for security reasons (I often run untrusted code with a temporary unprivileged user who can only write to ramdisks in order to drop, not raise my permissions). It is overall a bad idea to assume a specific permissions model like sudo or the existence of root or to assume a sudoed user has any particular privileges.
If you want to find out if you have permissions to perform an operation the best way is usually to simply try and do it then check errno for permissions issues if it fails or if it is a multi stage operation that must either all fail or all succeed you can check if an operation will work with functions like the POSIX access function (beware of possible race conditions here if permissions are actively being changed)
If in addition you need to know the real user behind the sudo you can use getlogin function which should work for any interactive session with an underlying terminal and would allow you for example to find who is 'really' running the command for auditing or find the home directory of the real user to save logs.
Finally if what you really want is to find out if a user has root access (Still a bad idea but less implementation specific) you can use getuid to check for a uid of 0 and thus root.