I don't think there is a simple way to distinguish, and that's kind of amazing when you think about it. There is no way to say something like "there is a half inch layer of fat here." Skin calipers were popular maybe 30-40 years ago to do this, but they turned out to be wildly inaccurate. Most intriguing to me is we no longer have an objective standard for abdominal strength. We used to do Roman Chair sit ups holding weights, but I don't think anyone would do them now. I think 15 reps with 25 pounds was considered strong. Holding a plank for 45 seconds is considered good, but only if done perfectly. You need a spotter to verify and this is as much a test of endurance as strength. If you are free weight squatting and deadlifting your body weight for reps, your abs are probably well developed. "...when I relax, it seems that I have a big gut..." sounds like a posture issue - check out lordosis and pelvic tilt. A six pack is formed by non muscle fibers that hold in the ab muscles - you may not have them. Skin tightness is a huge, under discussed part of 6-packiness. "Sometimes after running or weightlifting..." I wonder if you are retaining beer or salty snacks at other times. Water retention is a problem for body builders prior to competition. A good set of erectors and obliques will help stretch your skin across your abs (assuming your skin is young). The Body Mass Index (BMI) seems to be holding up as an objective fat standard. You should have a good score for health's sake, and it seems unlikely you can have a high score and showy abs at the same time (you could still have powerful abs however). Hope this helps.