$\DeclareMathOperator\GL{GL}\DeclareMathOperator\Frob{Frob}$In his paper, Duke paper, Serre consider continuous, odd Galois representation $\rho: G_{\mathbb{Q}}\longrightarrow \GL_{n}(\overline{\mathbb{F}}_{p})$ where $p$ is a rational prime. Roughly, (I don't understand much French except for the help from Google translation) Serre claims (section 1.3) that
$\det(\rho(\Frob_{l})) = \varepsilon(\Frob_{l})\omega^{k}(\Frob_{l})$ for all prime $l\nmid pN$ where $N$ is defined as the level of the representation (with an explicit formula given in the paper) and $\epsilon$ is a Dirichlet character and $k$ is some positive integer.
This seems to be standard since other papers cited it without reproving and I could not find any reference for the proof. In particular, my questions are:
Where can I find a proof for this.
What exactly is $\varepsilon$, in some paper, there is the claim that $\varepsilon$ is the unique quadratic character mod $p$ ramified only at $p$, and I do not understand where this comes from?
How can one find $k$.
For motivation, I think $\det(\rho(\Frob_{l}))$ is an important invariant to compute since, for example, it appears in the attachment equation that associates these representations with modular forms.
Thanks in advance for any insight.