I'm sure evryone is a little different, but here is how I tell the difference- though it can be tricky because when IBD is active, I find that IBD is amplified (pain feelings / disocmfort are exagerated) by the IBS. That is, IBS makes us sensitive to what might not be such uncomfortable/painful feelings if it were IBD without IBS.
To answer the question, though, I find that active IBD creates a sharpness and throbbing level of pain that doesn't occur with IBS symptoms. I am sure we all "know" a couple dozen different types of gut pain, our familiar friends (enemies), and most of the time my instincts work well in telling me whether it si IBS vs IBD.
Pancolitis dx'd 1986, full med-free remission 88-97
Flaring or simmering ever since
10 20 17.5 15 12.5 mg pred, 100 mg Imuran
Probiotics (Primal Defense and others), TSO
Turmeric/circumin, fish oil, many vits/minerals
Lower-carb version of Specific Carb Diet (SCD)