How can you determine (approximately) a serving size if you do not have a nutrition label to read?

What are you serving? Drinks? Ice Cream? Vegetables?

How about Googling the specific substance and adding the key word, "calories," if that is what interests you? For example:

https://www.google.com/search?client=safari&rls=en&q=vanilla+ice+cream+calorie+count&ie=UTF-8&oe=UTF-8

Or you could use "nutrition label" as part of the key words.