Isosceles triangle - equal side symbol

this line you can draw with content of node:

\documentclass[12pt,table]{beamer}
\usepackage{pgfplots}
\usetikzlibrary{quotes}

\begin{document}

\begin{frame}
    \begin{figure}
    \centering
\begin{tikzpicture}[scale=0.7]
\draw   (0,0) -- node[sloped] {$|$} (-4,-3)
        (0,0) -- node[sloped] {$|$} ( 4,-3) 
        (-4,-3) -- (4,-3);
\end{tikzpicture}
    \end{figure}
\end{frame}
\end{document}

According to @Bernard's comment, one can declare the mark with tkz-euclide package (see page 77).

The following are two (slightly) different ways to represent isosceles triangles with provided coordinates and desired marks:

\documentclass[12pt,table]{beamer}
\usepackage{tikz,pgfplots}
\usepackage{tkz-euclide}

\begin{document}

\begin{frame}
\begin{figure}[!htbp]
\centering
\begin{tikzpicture}[scale=0.7]
\node (A) at (0,0){};
\node (B) at (-4,-3){};
\node (C) at (4,-3){};
\draw (0,0) -- (-4,-3);
\draw (0,0) -- (4,-3);
\draw (-4,-3) -- (4,-3);
\tkzDrawSegments(A,B C,B A,C)
\tkzMarkSegments[mark=|,color=red,size=6pt](C,A A,B)
\end{tikzpicture}

\bigskip

\begin{tikzpicture}[scale=0.7]
\tkzDefPoint(0,0){A}
\tkzDefPoint(4,3){B}
\tkzDefPoint(8,0){C}
\tkzDrawSegments(A,B C,B A,C)
\tkzMarkSegments[mark=|,color=red,size=6pt](C,B A,B)
\end{tikzpicture}
\end{figure}
\end{frame}

\end{document}

Dependeing on what you need, you can change character | by || or even |||, and also customize the color of the marks, as images above.

You can also use the following code (I took it from wikipedia here, and then modified it)

    \usepackage[pdftex,active,tightpage]{preview}
    \usepackage{tikz}
    \usetikzlibrary{positioning, calc}
    % Begin tikz
    \begin{tikzpicture}[scale=0.8]
      \coordinate[label=left:$A$]  (A) at (-1,0);
      \coordinate[label=right:$B$] (B) at (5,0);
      \coordinate[label=above:$C$] (C) at (2,3.464);

     % angle CAB
      \begin{scope}[shift={(-1cm,0cm)}]
        \draw[fill=green!30] (0,0) -- (0:0.6cm) arc (0:50:0.6cm);
      \end{scope}
     % angle CBA
     \begin{scope}[shift={(5cm,0cm)}]
        \draw[fill=green!30] (0,0) -- (-180:0.6cm) arc (180:130:0.6cm);
     \end{scope}

     % drawing triangle
     \draw [line width=1.5pt] (A) --  (B)
                              (A) -- node[sloped] {$|$} (C)
                              (B) -- node[sloped] {$|$} (C);
  \end{tikzpicture}

and you should get the following picture: