I don't know much about Yanmar's, but a lot of front diff's that don't have a dip stick or sight glass will have a plug to remove on the front side of the axle. You place the tractor on a level surface and and remove the plug. If fluid runs out, you are good, if not, you open the top fill port, and and pour some in until it starts running out the plug hole, and stop and put in the plug. Hopefully someone will come along soon to either confirm or deny my hypothesis.
I'm with k0ua on his statement.
It's very common on all sorts of gearboxes - I had 2 Volvo 140 series cars and a P1800 with Borg Warner manual trannies that all did the same.