It can be hard to tell when you've reached "stiff peaks" with Swiss meringue because of how heavy it is compared Italian and French meringue. It doesn't triple in size the way the others do. Globs of it might not stand upright on the whisk, they'll kind of lean over. But they shouldn't lean and keep leaning. They'll lean and stay that way. A trick I learned in pastry school is to knock the whisk attachment against the sides of the bowl to get most of the meringue off of it — if you see lots of little peaks sticking up off the wire tines, congrats, you've got stiff peaks. Swiss meringue that has reached "stiff peaks" will hold its shape once piped. So if you pull the whisk up and the meringue is flowing off the whisk, you don't have stiff peaks.