It has been estimated that up to a third of the stamps of the NB 1888 issue in any one collection would be frauds. It was about 15% in my case. There are broadly 3 types of forgeries with this issue. In the first type we have 3 examples above which should be easy to recognise. The printing is coarse and blurred with different shades of colour. The paper is thicker and the perforations are inconsistent and variable. The other 2 types can be difficult as they were well printed. They were attributed to Rene Careme of Paris and Francois Fournier of Geneva. They were similar as Fournier was thought to have made use of the printing plates from Careme. However, Careme's creations were said to be of a better quality but in practice it is difficult to differentiate. The main differences from the genuine stamps include type of paper used and the shades of colour. Most of them are also pristine looking with full undisturbed gum. Genuine stamps very often have cracked gum. Also stamps with an irregularly thick Labuan type bar cancel are more likely to be from Carame.
I have highlighted the differences from the genuine stamps which are all on the left. The main feature in the half cent stamp is the extra stroke in the uppermost chinese and the left corner ornaments centre more to the right. I have also noticed in my collection, the forgeries were all in the magenta stamps and none in the rose which is the cheaper shade.
With the 2c stamp, the shade is a lighter brown. The left bottom ornament is more centred to the left and the 2 is closer to the upper frameline.
This is probably a Fournier with the usual irregular bar cancel. It is perhaps a lighter shade but this stamp is rather faded. The main thing is the crew cut appearance of the lion head.
The 10c does not have a dot flaw just above the letter TH and the CENTS has variable thickness. The T is shorter and the S has a sharper tail stroke. The colour has a greyer shade. Update I have updated the image. My apologies for highlighting the wrong flaw previously. This is a small dot which is constant in the genuine stamps.
The only genuine item here is the half cent. I would say most of the imperfs of this issue are forgeries unless proven otherwise. They are all pristine looking with full shiny gum. Whole sheets or high multiples are often available. The 3c has no rope from the mast to the flag pole. The left bottom ornament is centred to the right. The 6c has 2 parallel ropes from the deck that does not converge to the mast. The 8c has a thick and distorted rope from mast to flag pole. The ornaments on the left are centred more to the left. The upraised paw of the lion almost touches the frame above.