= Solution
A <symmetry> is a transformation of a physical system that preserves its laws: it maps admissible solutions and consistently transformed data to admissible solutions. The central <symmetry> of <special relativity> is <Lorentz invariance>, expressing equivalence of <inertial frames> and preservation of the <Minkowski interval> and the universal <speed of light>. With spacetime translations included, the spacetime <symmetry group> is the <Poincaré group>.
A practical sufficient check is to express an equation as an equality between <Lorentz tensors> of the same type, with every repeated upper/lower pair correctly contracted and the same free indices on both sides. Every ingredient, including sources and background fields, must have its stated <Lorentz transformation> rule. For example, $p^\mu p_\mu=-m^2c^2$ is a <Lorentz scalar> equality, and $dp^\mu/d\tau=qF^\mu{}_{\nu}U^\nu$ equates <four-vectors>. Such manifest <Lorentz covariance> makes the form of the equation the same in every <inertial frame>.
This is a sufficient test when the transformation rules are genuine physical ones, not a guarantee from writing index symbols. A fixed preferred <four-vector> could make an equation formally covariant if it too were transformed, while its chosen physical background breaks <Lorentz invariance>. Conversely, an equation written in three-dimensional components can be fully consistent with <special relativity>, as the <Lorentz force> is, even when the covariance is not manifest.
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