The thing that actually matters is knowing which clients are worth keeping around. If someone's pressuring you to hide AI involvement, they're already signaling they know it's dodgy - otherwise why hide it? That's your real warning sign, not the AI itself. Walk away from those gigs before you end up in a situation where you're legally liable for misrepresentation.
For clients who are genuinely just trying to optimize workflow, there's a middle ground. You can work with AI-assisted content as a starting point and be transparent about it - disclose it where it matters (to their stakeholders, in their own communications if required by law), but don't pretend you hand-wrote every sentence from scratch. Most decent clients actually appreciate knowing you're efficient. The ones who freak out at the idea of disclosure? Those aren't long-term relationships anyway. You'll spend more time managing their anxiety than you would just declining the project.
The contracts angle that others mentioned is solid, but go one step further: ask clients upfront what their industry or audience actually requires. Some sectors have real disclosure requirements. Others just have anxious decision-makers. Once you know which is which, you can price accordingly and decide if it's worth your time. If a client wants fast, cheap content with plausible deniability built in, that's not a copywriting job - that's a liability waiting to happen.