How do I get the position of the Turtlebot at current time and check if that pose is exactly same to the pose in the plan provided by the planner?

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Generally, the navigation stack get the robot position from odometry, which comes from your robot encoders. In the simulation environments(e.g. Gazebo), the odometry is always right, and your robot position is always right, so your robot is always following the plan.

But in the real world, the odometry is only right for a short time, and then both the position and orientation will drift. So the real robot will not follow the plan generated by the planner. To know if your robot is following the plan, you need additional position methods, such as GPS and amcl, which can tell you the robot's position relative to the 'world'. With these info, you can know if the robot is following the plan.