How to differentiate an abscess and a inflammation due to new piercing?

How they look & time: With piercing, it is common to have inflammation (redness surrounding the area and some pain) as this is a normal reaction of the body. If infection develops ( after inflammation usually) the pain might become worse and an abscess (collection of pus) build up and you might feel the area getting filled and some discharge comes out

Fluid under the skin: An abscess would typically feel like fluid under the skin ("fluctuant") whereas inflammation from a piercing would just be red and swollen but would not have a feeling of fluid or pus.

Abscess: Here is a NZ based dermatology site with some images and general advice around abscess: http://www.dermnetnz.org/topics/cutaneous-abscess/ It also has some links to piercing reactions: http://www.dermnetnz.org/topics/jewelry-allergy/ If any doubt/painful ++/unwell I do recommend seeking urgent medical attention as skin infections can be serious if the diagnosis is delayed.

Piercing : Inflammation due to piercing causes slight redness and slight tenderness to touch at piercing site whereas abscess would cause redness spreading to surrounding areas, discharge , tender even on slight touch and pain getting worse .