\documentclass{article}

\usepackage{agda}

% Make sure that cluster counting is activated.
\begin{code}[hide]
  {-# OPTIONS --count-clusters #-}
\end{code}

\begin{document}

\AgdaPostulate{A} and \AgdaPostulate{B} should be aligned:
\begin{code}
  postulate
    +̲+̲+̲+̲+̲+̲  A
            B : Set
\end{code}

The two \AgdaKeyword{field} keywords should not be aligned:
\begin{code}
  record +̲ : Set₁ where  field C : Set
                          field D : Set
\end{code}

\end{document}
