This usually happens when I'm writing to developers or other IT professionals.
The context would be something along the lines of ". . . measured at 100 MB/s (and is increasing)."
On paper or a whiteboard, I've always drawn a bold up/down arrow.
This situation mostly shows up when I type comments in email, code or revisions. I didn't not know if there was a symbol or shorthand in Unicode that could help out.