newcommand{qitem}{item?} %For the list of questions
begin{enumerate}
qitem Some question
end{enumerate}

documentclass{article}

makeatletter
defqitem{begingroupcatcode`^^M=12 qitem@}
bgroupcatcode`^^M=12 %
  gdefqitem@#1^^M{item #1?endgroup}%
egroup
makeatother

begin{document}
begin{enumerate}
qitem Some question
qitem Some other question
item Not a question
end{enumerate}
end{document}

documentclass{article}
newcommandqitem[1]{item #1?}
begin{document}
  begin{enumerate}
    qitem{Is this a question}
    item This is an answer.
    qitem{Was that a question}
    qitem{Can this question not be answered}
  end{enumerate}
end{document}

documentclass{article}

usepackage{collectbox}

% Say we want to call this command for each item
newcommandquestion[1]{#1?}

makeatletter
newifif@openbracket

% Define environment similar to enumerate where item's are terminated with "?"
newenvironment{enumerateq}%
  {%
    enumerate
    letorigitemitem
    defitem{%
      if@openbracketegroupendgroupfi%
      origitem
      begingroup@openbrackettruecollectbox*{question}bgroup
    }
  }%
  {if@openbracketegroupendgroupfi%
   endenumerate}
makeatother

begin{document}
  begin{enumerateq}
    item Is this a question
    item Was that a question% The new line is part of the content of the item. Comment out to suppress space.
    item Can this question not be answered%
  end{enumerateq}
end{document}