'Try this test. You'll need a friend to hold the bike up, or set it on a wind trainer. Sit on your bike with your hands on the handlebars and the crank arms horizontal. If you have a drop bar, hold the bar out on the brake hoods. Try taking your hands off the bar without moving your torso. If it's a strain to hold your torso in that same position, that's an indication of the work your arms are doing to hold you up.'
Problem with that is you need to move the saddle way back (usually beyond what the seatpost and saddle rails allow) for you to be able to balance your torso without straining. At least that's been my case every time I tried to do this.