Am I working my abs if I don't feel sore?

not necessarily! it's a possibility that you aren't engaging your core as much as you could be, but most of the time you get sore if you do something new or different - it throws your body off and that's what causes doms. you don't have to be sore after a workout to know you out in work.
I would recommend more strength training and compound exercises like squats and deadlifts to add to your ab strength too

Ab pain isn't really an indicator of how hard the muscles have been worked. You actually work your core every day just by moving around so it's one of those muscle groups (like your legs) that get worked every single day and are just used to it. However, cardio by itself is not going to focus on your abs as the core muscles are just going to work as stabilizers. You're seeing changes in your abs because it's likely you're losing fat in that area which will make your abs pop more or at least shrink the circumference of your waist. I can kill abs and not be sore although I'll feel really tight. They're just used to it. Compound exercises are great for core but you can also do more site specific exercises (crunches, planks, leg lifts) to focus specifically on them.

I will rarely ever feel sore in my abs, even after doing an intense ab workout.

The only time I encounter REAL soreness in my abs is after hitting a new record on deadlifts.