First of all, I apologize for how long it’s taken me to answer!
Hanfu generally all share certain basic characteristics: a y-shaped collar (交領右衽), usage of ties rather than buttons, a central seam down the torso of the upper garment (中縫), tying/fastening the garment at the waist (束腰), attachment of the sleeves around midway down the upper arm rather than at the shoulder, and flat cutting (as opposed to the use of a dress form). Of course, there are exceptions—對襟襦裙 doesn’t have a y-shaped collar, for example, and 披風 following the style used in the Ming dynasty uses metal buttons for fastening—but something considered hanfu would adhere to most, if not all, of these guidelines. There are other things related to fit, cut, and the like, but that’s something I wouldn’t be able to easily describe to you. If one has looked at a lot of material related to hanfu, though, then things like that become very obvious when they are done differently.
So as soon as an article of clothing more or less strays from these basic characteristics, it would no longer be considered hanfu. Anything made using a dress form (dummy) would not be hanfu, as flat cutting (平面剪裁) is one of the essential characteristics of hanfu. Using things like zippers, Mandarin buttons, etc. would also be a sign that the article of clothing in question is not hanfu.
The pictures below are a perfect example of an article of clothing that is mostly inspired by hanfu but is not actually hanfu. We would call this 漢元素時裝, or contemporary clothing with elements of hanfu.
Usage of lace itself would generally indicate that something is not hanfu, but here one can see that the lace panel has interfered with the way the upper garment should be cut. So while this resembles hanfu superficially, it is not hanfu.
Another example is below.
Here, the cut of the sleeves is an indication that the upper garment is not hanfu but rather 漢元素時裝. (And of course, pairing it with a modern skirt renders it no longer hanfu as well.)
I hope this answered your question!
More examples of 漢元素時裝 can be found here. The hanfu article and the 漢元素時裝 article at Baidu Baike give fairly good overviews, so those who can read Simplified Chinese may want to take a look.
Source for the first set of photos.
