2010 Craftsman 54" YT4500 Loss of drive

The mower will not go uphill and at times barely moves on flat ground. Seems to have more power in reverse than forward (but not by much). There is no noise or any indication that the belt is slipping.

Think it's the Hydrostatic Transmission gone out?

Couldn't find where to check for fluid to see if it was low.

Any ideas what I should check?

Thanks.