I only knew this angle symbol "$\angle$", which is usually used to represent acute angles. But now I have accounted a problem, where I wanted to represent the reflex symbol of $\angle ABC$.
I know, I can simply say "The reflex $\angle$ of $\angle ABC$" or "2$\pi$ - $\angle ABC$", but is there a symbol for reflex angle, just for interest?
If allowed to handwrite (or in this case to use a drawing software), I would use something similar to 
$\large ABC$. Not sure if this is standard notation or if there is some $\LaTeX$ glyph for that.
The point is that you can invent the symbol you want and use it consistently... the only two matters are
This question has always concerned me. To my knowledge: no, there isn’t.
Unicode has angle ∠, right angle ∟, and oblique angle ⦦, but nothing for reflex angle.
$\LaTeX$ has angle $\angle$ and (wide)hat $\widehat{B}$.
Just today I taught my students that $\angle ABC=A\widehat{B}C\leq180$ and if you want to refer to the larger angle at the vertex you literally just say/write reflex $A\widehat{B}C$. I think reflex $\angle ABC$ is a little contradictory but if you imagine an “of” then it makes perfect sense—reflex of $\angle ABC$.
One of my students suggested $A\check{B}C$ which I think is genius. But it’s only good if you’re already using the (wide)hat $\widehat{B}$ notation.
My advice (currently) is to assume $\angle ABC$ or $A\widehat{B}C$ is oblique (acute or obtuse) and specify reflex $A\widehat{B}C$ or reflex $\angle ABC$ if you need to.
It doesn’t feel like a satisfying answer but it’s the one I’ve decided to live with.
EDIT
\widecheck looks better for the upside down hat but doesn’t render here on SE.
