Highlight footnote in the original tex file?

In TeXstudio, you can modify the Language Definition.

Note: We expose the language specification to you as end-user to give you more flexibility in adapting TeXstudio to your needs. But you should take it as is, because we don't have the capacity to give support here.

For your case:

1. Download tex.qnfa
2. Create a folder languages in your settings directory and place the file therein.

3. In the simplest case, just treat \footnote like \todo: Add the line below to the existing <context id="comment/todocmd" ...>

   <context id="comment/todocmd" format="commentTodo">
        ...
        <start parenthesis="todo:open" prenthesisweight="12">\\footnote\{</start>
        ...
   </context>
   

Restart TeXstudio. The result should look like this:

Of course, you may copy the comment/todocmd context and create your own context if you need more control.

Rather than using syntax highlighting, why don't you improve readability by moving your footnotes out of the main paragraph?

With the sepfootnotes package you can put (longer) footnotes outside paragraphs.

Here are a couple of examples:

Example 1: footnotes separate from paragraphs

\documentclass{article}
\usepackage{sepfootnotes}
\begin{document}

\sepfootnotecontent {a} {See Tom (2002) for a similar view.}
\sepfootnotecontent {b} {See Jim (2001) and Frank (2002).}
\sepfootnotecontent {c} {Author (forthcoming).}

The film Star Trek is interesting.\sepfootnote{a}
But some have argued that it is not well directed.\sepfootnote{b}
I've explained elsewhere that this is not right.\sepfootnote{c}
In this paper, i will blah blah blah.\footnote{dadada.}

\end{document}

Notice that you can use \sepfootnote for long notes, and keep using standard \footnote for short notes (like your "dadada" note) if you want to.

Example 2: footnotes in a separate file

Better still, here is how to move your footnotes to a separate file using \input.

doc.tex

\documentclass{article}
\usepackage{sepfootnotes}
\input{notes}
\begin{document}

The film Star Trek is interesting.\sepfootnote{a}
But some have argued that it is not well directed.\sepfootnote{b}
I've explained elsewhere that this is not right.\sepfootnote{c}
In this paper, i will blah blah blah.\footnote{dadada.}

\end{document}

notes.tex

\sepfootnotecontent {a} {See Tom (2002) for a similar view.}
\sepfootnotecontent {b} {See Jim (2001) and Frank (2002).}
\sepfootnotecontent {c} {Author (forthcoming).}

Example 3: shorter footnote commands

And you can make your text even more clean if you define shorter footnote commands by using \newfootnotes, which is also provided by the sepfootnotes package.

For example, in order to typeset footnotes using \anote and \anotecontent, instead of the default \sepfootnote and \sepfootnotecontent, you just need to declare \newfootnotes{a} in the preamble. Like this:

\documentclass{article}
\usepackage{sepfootnotes}
\newfootnotes{a}
\input{notes}
\begin{document}

The film Star Trek is interesting.\anote{a}
But some have argued that it is not well directed.\anote{b}
I've explained elsewhere that this is not right.\anote{c}
In this paper, i will blah blah blah.\footnote{dadada.}

\end{document}

notes.tex

\anotecontent {a} {See Tom (2002) for a similar view.}
\anotecontent {b} {See Jim (2001) and Frank (2002).}
\anotecontent {c} {Author (forthcoming).}

Well, with a little pretty printing of your TeX code I think you can better see where footnotes starts and ends. Then there is no tool needed I think ...

See the following code example (I used a longer text in one of your footnotes):

\documentclass{article}
\begin{document}
The film Star Trek is interesting.\footnote{See 
  Tom (2002) for a similar view. Dummy text for a longer footnote to 
  show the line breaks I do in the \TeX{} code.} 
But some have argued that it is not well directed.\footnote{See 
  Jim (2001) and Frank (2002).} 
I've explained elsewhere that this is not right.\footnote{Author 
  (forthcoming).} 
In this paper, i will blah blah blah.%
\footnote{dadada.}
\end{document}

Because footnotes should follow directly the sentence or the word they refer to I use the following pretty printing: After the first word of the footnote I break the line by my own, insert two blanks in the following line and continue the footnote. The result is that I can easyly see the start and ending of written footnotes.

In the case you have only one word in the footnote (your last given example) I write an % after the dot of the sentence (to avoid that there could be a inserted blank) and write \footnote{dadada.} in the next line, followed by a line break.