Is there a symbol for 'Has Units of'? Is there a shorthand way to note that a variable has certain units?
I vaguely recall using a boxed equals sign '[=]' as in: 
x [=] feet

But I'm not sure if that's an accepted standard or if I made that up when were doing dimensional analysis in 10th grade. 
Thanks!
 A: I would argue that the box notation above would be used in the following context, for velocity $v$
$$[v ] = \frac{[L]}{[T]}$$
Where 
\begin{align}
[L] &= m \\
[T] &= s
\end{align}
For units of length, $m$ in metres and time, T in seconds.
A: In addition to boxing the variable, $$[SA] = [m^2],$$ if you put your variable declarations before your work, in the heading, you can usually get away without the boxes, $$ D = \frac{kg}{m^3} $$ I've seen this done in several papers, sometimes bolded. As long as it is in the heading to avoid confusion with the rest of the work, such that it is clear that $kg$ and $m$ are not variables.
A: I was taught in physics that ): meant "that is, in units of", but I have not been able to find any online source of this. It's use would be after a unit-less calculation to specify the units of the answer.
$$... \frac{10}{5} = 2 \textrm{ ): m/s}$$
This example would mean some form of calculation resulting in 2 m/s, but maybe the units were "obvious" or for some reason excluded, typically in hand-written notes, and there was a need to clarify the resulting units at the end.
Other than that, as other answers have pointed out, square brackets are used to denote the units of the variable and not the variable itself:
$$ v = \textrm{m/s}$$
And please follow convention that variables are italized but units are not. $l$ could denote a variable for lenght, but its unit is m which is short for metre. $m$ would be interpreted as a variable. 
Units should only be in italics if the whole sentence is italized, such as in this sentence, where I explain that time is usually written in equations as t with unit s.
