I beleive you
how can you tell?
For me personally, I take half a glimpse and don't even think about it, I just know, sort of like if you look at a face you know if you're looking at a man or a woman, but if you were to be asked how you know you'd have to stop and think about it. Is it the shape of the eyes? Mouth/lips? Ratios of distance between facial parts? You likely don't even know what the diagnostic features are, but you can still do it, right?
But if you wanted to explain to someone else how to definitively diagnose a person as a man or a woman, there are bits you'd go for which you wouldn't personally use to determine a person's sex.
So, what I'm mostly using is head shape (I won't try to articulate this with words, but the shape is different), eye size (Children's Pythons have proportionately much larger eyes), colour, proportionate scale size and several others. As a beginner you probably won't be able to use these just yet.
So, the specific diagnostics you can most easily use are probably scalation. The scales on the parietal area are larger and shaped differently on an Olive, and perhaps the most obvious single diagnostic for you to use (it's impossible for me to see in this picture) is that Olives have one, rarely two loreal scales while Children's Pythons always have more than two (they're also shaped very differently but you can just use the number). The loreal scales are the ones between the eyes and the nose, bordered below by the labials (the lip scales) and above by the prefrontals. You might have to look up these terms (do a web search for 'snake head scalation' and perhaps include terms like frontals, labials, loreals, supraoculars and you should find what you need, including labelled diagrams). On some snakes it can be slightly difficult to distinguish between the loreals and prefrontals but in both Olives and Children's they are quite distinct.
Again, this is not the method I would usually use to determine if a snake is an Olive or Children's and it's sort of the equivalent of taking off an otherwise naked person's underpants in order to detemine their sex, but like inspecting what is between a person's legs, it is a completely unnecessary but most easy and definitive way to determine the answer. There are other definitive ways to distinguish between them but I think you'd find the others more difficult.
Of course, there is a radical difference in size between Olives and Children's.
I definitely should have said in my first post, that's an outstanding example of a patternless Children's Python, they're quite popular and I'm personally very fond of them. You're fortunate to have such a fine example! Beautiful snake!