The progress of the task can be found inside the metapanel widget of any task form, under the title of the task. It shows a grey bar partially or totally filled in by a green bar (depending on the progress). When progress is 100% the green bar completely overlays the grey bar. The progress percentage could also be shown on the right side of the widget, rounded to the nearest integer and followed by the word 'complete', but that depends on the widget configuration.
The progress is calculated using the following formula:
Which means that, in order to reach 100% on a task, all the values of the output (editable) properties must be filled in. A task that is not linked to any parts or properties has the progress set to 100% by default.