Checking if an integer is a member of an integer list

You can use do a comma-separated list parser to loop through the list of items and check if the given item is present in the list. Since the comma-separated list can be read by using a delimited macro, this can be done expandably, allowing you to plug the macro into \fpeval to get the syntax you want.

Using expl3 (you are loading it with xfp anyway) something like this would solve the problem:

\ExplSyntaxOn
\prg_new_conditional:Npnn \afp_int_ismember:nn #1#2 { p, T, F, TF }
  { \__afp_ismember_loop:nw {#1} #2 , \q_recursion_tail , \q_recursion_stop }
\cs_new:Npn \__afp_ismember_loop:nw #1#2,
  {
    \quark_if_recursion_tail_stop_do:nn {#2}
      { \prg_return_false: }
    \int_compare:nNnTF {#1} = {#2}
      { \use_i_delimit_by_q_recursion_stop:nw { \prg_return_true: } }
      { \__afp_ismember_loop:nw {#1} }
  }
\ExplSyntaxOff

The code above defines a conditional \afp_int_ismember:nn(TF), whose first argument is the item to check, and the second is the comma-separated list. The macro starts by expanding \__afp_ismember_loop:nw: this macro takes the item to be tested (#1) and the first item in the list, delimited by a , (#2). The macro tests the equality of #1 and #2 using \int_compare:nNnTF and issues \prg_return_true: if they are equal or calls \__afp_ismember_loop:nw for the next item. If the end of the list is found (i.e., \q_recursion_tail is grabbed), then the function issues \prg_return_false: because no match was found for #1.

The code above can be tweaked to replace \int_compare:nNnTF by a generic equality comparison function. If you do that you can define wrappers around \__afp_ismember_loop:nw to create ismember functions for different data types. The code below does that and defines two functions: \afp_int_ismember:nn(TF) (using \int_compare:nNnTF) and \afp_str_ismember:nn(TF) (using \str_if_eq:eeTF). Doing that you can even test if I'm a member of the Beatles!

To use the function in \fpeval you just need the predicate form (\afp_int_ismember_p:nn) of the conditional function:

\fpeval{ \afp_int_ismember_p:nn {1} {1,2,3,4,5} ? 123 : 321 }

And the string version would also work here:

\fpeval{ \afp_str_ismember_p:nn {Phelype} {George,John,Paul,Ringo} ? 123 : 321 }

---

Full code:

\documentclass{article}
\usepackage{expl3}
\usepackage{xparse}
\usepackage{xfp}
\ExplSyntaxOn
% Core code for the membership test
\cs_new:Npn \__afp_ismember_loop:Nnw #1#2#3,
  {
    \quark_if_recursion_tail_stop_do:nn {#3}
      { \prg_return_false: }
    #1 {#2} {#3}
      { \use_i_delimit_by_q_recursion_stop:nw { \prg_return_true: } }
      { \__afp_ismember_loop:Nnw #1 {#2} }
  }
% Wrapper for testing integers
\prg_new_conditional:Npnn \afp_int_ismember:nn #1#2 { p, T, F, TF }
  {
    \__afp_ismember_loop:Nnw \__afp_int_isequal:nnTF {#1} #2 ,
    \q_recursion_tail , \q_recursion_stop
  }
\prg_new_conditional:Npnn \__afp_int_isequal:nn #1#2 { p, T, F, TF }
  {
    \int_compare:nNnTF {#1} = {#2}
      { \prg_return_true: }
      { \prg_return_false: }
  }
% Wrappers for testing strings
  % With expansion
\prg_new_conditional:Npnn \afp_str_ismember:ee #1#2 { p, T, F, TF }
  {
    \__afp_ismember_loop:Nnw \str_if_eq:eeTF {#1} #2 ,
    \q_recursion_tail , \q_recursion_stop
  }
  % Without expansion
\prg_new_conditional:Npnn \afp_str_ismember:nn #1#2 { p, T, F, TF }
  {
    \__afp_ismember_loop:Nnw \str_if_eq:nnTF {#1} #2 ,
    \q_recursion_tail , \q_recursion_stop
  }
% Sample commands
\NewExpandableDocumentCommand { \IntIsmember } { m m }
  {
    \afp_int_ismember:nnTF {#1} {#2}
      { #1~is~member~of~`#2' }
      { #1~is~\emph{not}~member~of~`#2' }
  }
\NewExpandableDocumentCommand { \StrIsmember } { m m }
  {
    \afp_str_ismember:nnTF {#1} {#2}
      { #1~is~member~of~`#2' }
      { #1~is~\emph{not}~member~of~`#2' }
  }
\ExplSyntaxOff
\begin{document}
\IntIsmember{1}{1,2,3}

\IntIsmember{4}{1,2,3}

\StrIsmember{Paul}{George,John,Paul,Ringo}

\textbf{\StrIsmember{Phelype}{George,John,Paul,Ringo}}

\ExplSyntaxOn
\fpeval{ \afp_int_ismember_p:nn {1} {1,2,3,4,5} ? 123 : 321 }\par
\fpeval{ \afp_str_ismember_p:nn {Phelype} {George,John,Paul,Ringo} ? 123 : 321 }
\ExplSyntaxOff
\end{document}

Something like this? The macro name is inspired by the Mathematica command MemberQ, and the code comes from here. This solution does not require any packages.

\documentclass{article}
\newif\ifmember
\makeatletter% for \@for see e.g. https://tex.stackexchange.com/a/100684/121799
%from https://tex.stackexchange.com/a/498576/121799
\newcommand{\MemberQ}[2]{\global\memberfalse%
\@for\next:=#1\do{\ifnum\next=#2\global\membertrue\fi}}
\makeatother
\begin{document}

\MemberQ{1,2,3,4}{2}
\ifmember 2 is in list \fi

\MemberQ{1,2,3,4}{5}
\ifmember 5 is in list\else%
5 is not in the list\fi
\end{document}