Whilst Invert Level comparison is one way to check flow comparison in gravity drain/sewer networks, sometimes you can legitimately have adversely graded pipes......or more often the Invert Level data has occasionally been entered incorrectly to GIS/CAD where the pipe has been drawn in the correct direction, but either the manhole of pipe invert levels are incorrect and make the pipe appear to have a reverse grade.

Another reasonable way of checking overall line direction aligns with flow direction is to look for "Sources" and "Sinks". Generally these are the End Points on a network which can be found with TopologyBuilder to find which Manhole Nodes have one, and only one Pipe entering it. Alternatively sometimes the Sink can also be found with attribution if it happens to be network Nodes tagged as an Outlet or Pump Station structure.

Otherwise, the lowest of these end points is generally the "Sink", and all the other higher points are "Sources". The Source->Sink pathways can all be tested with ShortestPathFinder for a single pathway, dendritic network, which will generate "Paths" that will report any pipes that had to be reversed (and are probably drawn in the wrong direction) for a flow to move from each Source to the local network Sink.

An alternate, non-FME way to check line direction is to import the data into SWMM, which is perhaps one of the most widely used "freeware" gravity network analysis packages (https://www.epa.gov/water-research/storm-water-management-model-swmm), where you can generate artificial flows in the network and check if the pipe flow direction is in the opposite direction of the pipe line orientation, but also access to any commercial drainage/sewer network modelling software tends to have these gradient/flow direction checks built into the most popular civil engineering network analysis and design software such as StormCAD, InfoWorks ICM, MikeUrban to name but a few. If this a one off project to simply clean the data once, sometimes just using what other companies have already built to look at common specific drainage/sewer network questions can be easier! I even had one engineer show me his Excel checking/cleaning tool for this same problem many years ago, so it doesn't necessarily need FME!