Never noticed any dulling so far, but grapefruit is quite acid and the peel leaves sticky oil on the blade that dries rapidly. You might have a sticky rather than dull knife.
I'll push/pull cut on newspaper to see if there are any inconsistencies in the edge, then I'll roll the newspaper and cut on angle (in the air) and then thumb nail test. Also Carters three finger test is how I check during sharpening, it gives pretty immediate feedback.
Maybe I'm old school (and it looks kind of weird, but I do wear long sleeve shirts a lot), but I always try to shave some arm hair. I first try the tip, then the belly and finally the heel. That way I can test to see which areas need work. If all three sections will shave it clean, then I know that the knife is finished. In my defense, I'm used to working in wood-fired ovens and over flames, so half of my arm hair has always been burnt off anyway.