How many people does an engineer making 400k have to manage?
The first had a small team (< 5) of also highly paid senior developers like him to build and maintain a set of systems. I wouldn't call it "manage" so much as "coordinate colleagues". The second was working alone.
In both cases the work was difficult to do, difficult to share across a team (solve a few hard problems well rather than cycle through hundreds of small JIRA tickets) and highly value adding for the company which is why they were willing to pay.
Ideally zero as they're on an "individual contributor" track where they're focused on engineering. They may have other people who they mentor but no direct reports.
You check if they put their money where their mouth is. Generally, a (well-funded) company that pays its (senior) engineers the same as or more than its executives values engineering. To name two, I've noticed both Google and IBM immediately matching friends' competing offers no questions asked. I know two other engineers whose base, cash salary was over USD 400,000 / year. Cash is very expensive to managers, and enormously expensive to startups (equity less so, but still quite a bit) so it is a reliable signal.
A subtler way might be to check the flavour of engineering projects, and particularly their time horizon. Bringing up Google again, one SDE mentioned using machine learning to automate managing their infrastructure. That's the kind of risky, ambitious, long term project you want to look for.
I personally take mentions of intangible things as negative signals from experience. Something along the lines of if someone is trying to reassure you about something, it means they think it's an issue.