Aside from an oscilloscope with properly isolated inputs, the only reasonable option to probe line-side signals in a SMPS is a differential probe. The usual differential probe with 25MHz bandwidth will faithfully reproduce "square" waves with a ~15ns risetime which ought to be more than sufficient.
That said, I use two different scopes with truly isolated inputs for most of my work on switchmode power circuits - an ancient, but much beloved Tek THS720 and a newer but not as much loved AEMC OX7104. I also have a couple of B&K Precision 25MHz differential probes but they spend most of the time draining AA batteries (but do work well when used).