I'm no expert but with transmission oil there are normally two removable plugs. One at the the bottom to drain the old oil, and one part way up the gearbox casing for filling/topping up. They would either inject a measured quantity or fill until oil starts to dribble out of the hole. By removing this filler plug they can check the oil level is satisfactory and maybe take a small sample to check the oil is still in good condition. This might not be an objective scientific test, just a subjective check by an experienced mechanic that the oil looks clean and doesnt feel old or gritty when rubbed between the fingers. Much as you might do with engine oil on the dip stick.
Or they may just have checked the cars service history as recorded on the Honda database and found it was changed recently enough in time or mileage that its not yet due. Or checked the oil and the service record.
