No exact way, but an intuitive one, i´d say. You normaly know you factional units, so any other local one would need local barraks, so far so good. Now, the stronger, better, "heavier", the more elite a unit is, the higher level of the barracks are required. The last tier are, mostly, elite cavalry units, or those rare super units with cool stats/abilities, which require the 5th tier of the barracks. And, though it is quite cool to have superb and unique units to play with, the realismus aspect requires realistic behaiviour: if you want access to the local elite units, you have mostly no choice but to establish a client kingdom. The only thing i miss is that one has to build up all the local barracks line from scratch, while normaly a king would simply call for aid, and his allies would provide troops, so far available. I do see the nessesity of building up you own factional barracks in the subdued territories, though.