I think the easiest way is to just 'eyeball it,' that is, look at the pot of stew, or pan of roast meat and ask yourself, 'How many people are going to be at this meal? After serving everyone, will there be leftovers (if so, how many people could sit down and have a serving of this? All that adds up to the number of servings in the pot - just try to make the servings fairly uniform in size and not serve more for one person and less for another.
It's going to be a pretty close estimate. If you put in all your ingredients for the recipe and served the food, then divide the recipe by the number of servings you would provide. Does it matter if three people sat at the table and each ate huge portions (1/3 of the pot for each)? No, it doesn't matter in terms of estimating. Your serving calories will be much more than if you divided your pot into smaller servings (say six to the pot and you each ate a serving leaving half of the pot left for leftovers).