To establish whether you are training your “core” effectively it is first important to understand exactly what you are referring to. When most people talk about their core they are generally referring to their rectus abdominis aka the six-pack muscle. This makes sense because it is the most visible (assuming you have a level of body fat low enough to see it), however in reality your core is made of a number of other muscles, each with their own specific function - see diagram below.

The image above shows the primary muscles that make up your core. Each muscle has it’s individual function including: both generating and resisting rotation of the torso, flexion (bending) at the spine and flexion of the hip (bringing your leg up towards your torso). It is therefore essential that you include all of these movements into your training to ensure that you hit every muscle in your core. If you can incorporate multiple functions into the same exercise then even better e.g. a Star Fish Crunch. Below is a video of a short and very effective core training session that covers all of the necessary bases - give it a try!

Some of your core muscles e.g. the transverse abdominis are not visible, regardless of how low your body fat percentage is because they are located in deep inside your torso. With that being said If you are purely concerned with aesthetics then focus on the rectus abdominis, serratus anterior and external abdominal obliques. Although be mindful that as I mentioned unless your body fat percentage is low enough then you are wasting your time, because no amount of core training is going to give you the toned stomach you are after if you have a layer of fat covering it up. Instead focus on improving your nutrition, get yourself into a calorie deficit and start burning some fat - if you need help with this I can provide you with meal plans.