Heres how you find out (make sure your editor is open tough)
{
$base = "graphEditor1Window|TearOffPane|graphEditor1|formLayout37|frameLayout7|formLayout38|flowLayout7|";
for ($item in {"splineTanButton","clampedTanButton",
"linearTanButton","flatTanButton",
"stepTanButton","plateauTanButton",
"breakTanButton","unifyTanButton"})
print (iconTextButton -q -image ($base+$item)
+"\n");
}
Some of the icons are located inside maya binary itself. And the path depends on maya version