I don't think theres a scientific way like there is with WW, this is one of the reasons I have asked a friend to pick up the shopping guide for me. What I have done to help as well is create a table on word with all the foods that have passed through my lips that have syns, or foods I like with syns, I keep a note of the syns for future reference. Are you able to get onto the slimming world website, because theres a syns calculator on there - shame they don't bring out one you can buy and carry around in your handbag!
that would make it easier indeed. maybe you could suggest it to the heads of SW. make yourself a little bit of commission at the same time. A little computerised shopping guide that you could keep in your bag. Fab idea.
for me it is the impulse buying, for example i was so bored on Saturday at the footie tournament that i bought a Wispa and yes it made me feel better. I knew that the syns would be high but if i had had it in black and white in front of me i would probably have resisted temptation.