substract points on secp256k - result as moved to N-1
How to check that substract points cross the order of curve n?

You cant'; or at least, we hope you can't.
If you could, you could use that method to compute discrete logs.
Here is one approach; suppose you had a method that, given the points $[a]G$ and $[b]G$, would return you $[a-b]G$ and also informed you whether it &quot;crossed the order of curve n&quot;, that is, if $a &lt; b$.  Then, what you could do, given the point $[x]G$, you could use this method on the points $[x]G$ and $[\lfloor n/2 \rfloor]G$; that method would tell you if $x &lt; \lfloor n/2 \rfloor$.  If it turned out (for example) if $x$ happened to be larger, you could repeat this with $[x]G$ and $[\lfloor 3n/5 \rfloor]G$; and continue to do binary searching until you have recovered the value of $x$.
That means that you've just recovered the discrete log of $xG$ with only circa $\log_2(n)$ calls to your method; we certainly hope that we can't do that.
BTW: why do you care if &quot;the points cross the order of curve n&quot;?