Unfortunately you can't unless you know the serving size, exact ingredients that were used, their weights and the cooking methods. And these will vary from place to place so it's nigh on impossible for SW to say that if you eat this dish then it will definitely be this number of syns. You just have to make sensible choices and estimate the syns, or plan in advance to have a flexi day and estimate the syns. For your dish was it some sort of stirfry? If so then it would likely have been done in oil and you would need to allow for that - maybe 6 syns for 1 tbsp would likely cover it.
