The LaTeX command for check is \check{x} (háček / caron).
\check{x}Writing a paper? The table generator and the BibTeX cleaner are one click away, no account.
Write \check{x} inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
\check{x} is a math accent: it takes an argument and only works in math mode, while text accents such as \'e are separate commands.
Open this example in the live preview: it renders as you type, with no compiler and no signup.
\overbrace{x}^{n}\underbrace{x}_{n}\vec{x}\dot{x}\ddot{x}\acute{x}\grave{x}\breve{x}\mathring{x}\widehat{xy}\widetilde{xy}\underline{x}\hat{x}\tilde{x}Math accents in LaTeX: the full table or search all 350+ LaTeX symbols.
Use \check{x} in math mode; it needs no extra package. For example, $\check{x}$.
None: \check{x} is built into LaTeX, so no extra package is required.