Expect to receive with particular argument at least once

You can use Receive Counts:

expect(Logger).to receive(:log).with('foo').at_least(:once)

The approach is to allow calls to log, do your stuff, and then check that log has been called with the expected arguments, like this:

allow(Logger).to receive(:log)
Bar.foo
expect(Logger).to have_received(:log).with("foo")

RSpec calls this pattern Spies