How do we know graptolites were pelagic?
Basically, we know they were likely pelagic creatures because we find their fossils at essentially all water depths.

If they were benthic creatures, we'd expect their fossils to be limited to a set range of paleo water depths.  It's unlikely that a single benthic (i.e. living on the seabed) marine organism could live in both shallow water and ultra-deep water.

However, for pelagic (i.e. floating or free-swimming) creatures, we'd expect to find their fossils at a huge range of paleo water depths because they live in the water column and don't care where the bottom is.