One way could be to select all nodes on that face and do a NFORCE,ALL (Sum @ each node, nodal calcs. tool).
To select all the nodes on a face one could use say:
asel,s,area,,1,, ! ! Selects Area nr 1
nsla,s,1 ! Selects node on this area
Nforce,all for the list of reactions. One can view the largest one in the file/list generated.
Alternatively one can loop[ through all nodes and get the largest nodal reaction stored in MAXRF (example below, FZ reaction).