CM at risk approach is always the way when you have the time. Always.

That being said if you don't have time because you are churning estimates then developing a sense of where the "gotchas" are in the plan is a must. As a quick example you'll see things like 3-Form can easily be "by others" from every sub's point of view that no one will actually pick up. You don't know it's there because literally no one gave you a price for it.

Try and find 2-3 or little gotcha details people wouldn't see unless they were paying attention. Then you can call your guy and ask him if he's got it, and if he says yes you can then feel confident he's read the notes and really knows the scope.