Maybe
you can't. Everybody has their own risk tolerance, but at some point, everybody's going to have to draw a line. Maybe you're only storing passwords for local services, but almost all of the credentials in my password manager are for services run on some cloud. Even then, did you evaluate all of the code for each of those services? How about the compiler code or the chips? Dell shipped out machines with a hardware trojan in 2010.
I have separate instances for work and personal accounts, so one breach wouldn't affect the other. Since my passwords are distinct, the number of accounts that would actually be useful to them is minimal, and fraud response is a pretty important metric in deciding what companies I do important business with. Identity theft is a problem, but all of this is probably more likely to be leaked in some other database, like the Equifax hack, than through an account compromised in a password manager cloud storage breach.
My password manager being compromised would indeed be a huge time suck, but I don't think the long-term consequences would be any more severe than a few key individual accounts that are probably even more vulnerable. I think things like coordinated attacks where they social engineer their way through 2FA— which have been seen in the wild— to present a greater real-world concern.