Within command definition peek at next command

The underlying TeX primitive to peek ahead is \futurelet but that would not skip spaces, LaTeX wraps this in a space skipping loop as \@ifnextchar which does a \ifx test that the token is equal to the specified token so despite its name implying a "char" test, it can be used to test for any token.

For the particular case of \cmd the space skipping is not needed, as no space tokens are made after a command name but if \cmd took arguments and you needed to peek after \cmd{x} then this \@futurelet version comes in useful as it would skip spaces after the }.

\documentclass{article}

\makeatletter
\newcommand{\cmd}{\@ifnextchar\nofoobar{\@gobble}{foobar}}
\makeatother

\newcommand{\nofoobar}{nofoobar}


\begin{document}

\cmd this should say foobar% prints foobar

\cmd\nofoobar this should not say foobar % does not print foobar

\cmd   \nofoobar ideally this should also not say foobar % that is, spaces should be ignored 

\end{document}

expl3 has a very powerful mechanism for peeking at the next token; you can optionally remove the next token or ignore spaces (or do both actions).

In this case you can use a function of the \peek_meaning family:

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\cmd}{}
 {
  \peek_meaning_remove:NF \nofoobar {foobar}
 }
\quark_new:N \nofoobar
\ExplSyntaxOff

\begin{document}

\cmd (this should say foobar)% prints foobar

\cmd\hfill (this should say foobar)% prints foobar

\cmd\nofoobar (this should not say foobar) % does not print foobar

\cmd   \nofoobar (ideally this should also not say foobar) % that is, spaces should be ignored

\end{document}

Here ignoring spaces is irrelevant because there is none in \cmd \nofoobar, but there's also

\peek_meaning_remove_ignore_spaces:NF

and of course you can have the NTF and NT forms for specifying, respectively, different actions in the true and false cases or just an action for the true case.