Turboprop Propellers can be ducted for extra thrust. What are the criteria for determining that the engine is no longer a ducted propeller design, but a turbofan? Does there have to be a type of blade design, a fully separate nacelle/pod, number of blades? Or are all turbofans technically shrouded propellers with some augmented thrust from the turbine exhaust ?
They're pretty much all considered turbofans with a few exceptions from the '70s such as this experimental Britten-Norman Islander. They key difference is a prop can be considered a separate entity in a turboprop, but a fan is integrated into the engine.
Slightly longer answer
After a fair bit of thought and research I hope I've managed to come up with an answer, though to start with a disclaimer. The aviation industry is far too fond of merging and creating new words to describe new engine architecture. Take Open Rotor vs. PropFan for example.
To get technical about it, you could look at the similar/reverse argument about the difference between an open rotor turbofan and a turboprop, as addressed by the EASA.
If you look at Appendix 1: Open Rotor Definition (Page 86) of that document they outlined the following key differences
Open rotor module that cannot be distinguished as a separate entity
However the following was the agreed definition, which is still rather ambiguous.
A Turbine Engine featuring contra-rotating fan stages not enclosed within a casing
I think the first one is more key, the prop can be considered as a separate entity, attached to the front of a turbine. A fan is an integral part of the engine and cannot really be considered a separate entity, as its usually a part of the actual turbine shaft and has functional turbine component passing through the middle of it.
In general (there are often exceptions to any rule), a turboprop unit is likely to have a gearbox between the turbine and the propeller. The propeller is likely to be capable of varying its pitch. Also, the propeller may be a set of two contra-rotating propellers in the case of a high power turboprop. A turbofan engine is unlikely to have these features, although PW does have a GTF geared turbofan.