You can use CheckTokenMembership (http://msdn2.microsoft.com/en-us/library/aa376389.aspx) to check whether a user is in administrators group.  There is an example in the MSDN document.

The example will not work in Windows Vista. In Windows Vista, even if the user is in administrators group, the OS will create a filtered user token when user log on. The SID of administrators group is not included in the filtered user token. The full token is linked to the filtered user token, and can be retrieved with API GetTokenInformation with the new TokenLinkedToken information type.

The following is the sample code. You need to download Vista SDK to compile it. Please make sure you read the article in the reference section.

HRESULT IsUserAdmin(BOOL *pIsAdmin)
{
    int b;
    HANDLE hProcess = NULL;
    HANDLE hProcessToken = NULL;
    HANDLE hLinkedToken = NULL;
    BOOL fIsAdmin = FALSE;
    DWORD dwLength = 0;
    OSVERSIONINFO osver = {sizeof(OSVERSIONINFO)};
    HRESULT hr = S_OK;

    *pIsAdmin = FALSE;

    // get handle to our process token
    hProcess = GetCurrentProcess();
    if (!OpenProcessToken(hProcess, TOKEN_QUERY, &hProcessToken))
    {
        hr = HRESULT_FROM_WIN32(GetLastError());
        goto Exit;
    }

    // get admin SID
    char AdminSID[SECURITY_MAX_SID_SIZE];
    dwLength = sizeof(AdminSID);
    if(!CreateWellKnownSid(WinBuiltinAdministratorsSid, NULL, &AdminSID, &dwLength))
    {
        hr = HRESULT_FROM_WIN32(GetLastError());
        goto Exit;
    }

    // check to see if the current token contains admin SID
    if (!CheckTokenMembership( NULL, &AdminSID, &fIsAdmin))
    {
        hr = HRESULT_FROM_WIN32(GetLastError());
        goto Exit;
    }

    if (fIsAdmin)
    {
        // printf("The user is in admin group, and the process is elevated.\n");
        *pIsAdmin = TRUE;
        goto Exit;
    }

    // if the current token does not contain admin SID, it does not mean
    // that the current user is not admin. In Vista by default the token of
    // users in administrator group has the the admin SID filtered. We nee
    // to use the unfiltered token to do the check.
    if (!GetVersionEx(&osver))
    {
        hr = HRESULT_FROM_WIN32(GetLastError());
        goto Exit;
    }

    // XP and below, we are done.
    if (osver.dwMajorVersion < 6)
    {
        // printf("The user is not in admin group.\n");
        goto Exit;       
    }

// get handle to linked token (will have one if we are lua)
    if (!GetTokenInformation(   hProcessToken,
                                TokenLinkedToken,
                                (VOID*) &hLinkedToken,
                                sizeof(HANDLE),
                                &dwLength) )
    {
        b = GetLastError();
        if  (   b == ERROR_NO_SUCH_LOGON_SESSION
            ||  b == ERROR_PRIVILEGE_NOT_HELD)
        {
            // printf("The user is not in admin group.\n");
            goto Exit;
        }

        hr = HRESULT_FROM_WIN32(b); // a real error
        goto Exit;
    }

    if (!CheckTokenMembership(   hLinkedToken, &AdminSID, &fIsAdmin))
    {
        hr = HRESULT_FROM_WIN32(GetLastError());
        goto Exit;
    }

    if (fIsAdmin)
    {
        // printf("The user is in admin group, and the process is not elevated.\n");
        *pIsAdmin = TRUE;
    }
    else
    {
        // printf("The user is not in admin group.\n");
    }

Exit:
    if (hProcess)
    {
        CloseHandle(hProcess);
    }

    if (hProcessToken)
    {
        CloseHandle(hProcessToken);
    }

    if (hLinkedToken)
    {
        CloseHandle(hLinkedToken);
    }

    return hr;
}

Reference:

Teach Your Apps To Play Nicely With Windows Vista User Account Control