\ifdim command with multiple conditions

As Steven said, you can use

  \ifdim\mydim>615pt\relax
    \ifdim\mydim<650pt\relax
      in range%
    \else
      above 650%
    \fi
  \else
    below 615%
  \fi

If the code for the two out of range cases is the same and you don't want to duplicate it then you can structure it as

\ifdim\ifdim\mydim>615pt\mydim\else\maxdimen\fi<650pt\relax
  in range%
\else
  out of range%
\fi

Nest them.

\documentclass{article}
\newcommand\mytest{%
  \ifdim\mydim>615pt\relax%
    \ifdim\mydim<650pt\relax%
      in range%
    \else%
      above 650%
    \fi%
  \else%
    below 615%
  \fi}
\begin{document}
\newlength\mydim
\mydim=630pt\relax
\mytest

\mydim=500pt\relax
\mytest

\mydim=700pt\relax
\mytest
\end{document}