I've seen it a couple of times in divers (not myself), but I've had to deal with it and can say the following from my experience, and in response to your questions.
- it looks like a skin rash, but is not as bright in colour. It's not on the surface of the skin and as consequence is darker in colour than an allergic reaction. In both cases, it was spread over the upper torso and if I had to give a location for the cases I witnessed it would be "below the nipples and above the belly button".
- there is no swelling of individual points. It definitely could not be mistaken for insect bites.
- neither case was painful, but one did itch. Both cases displayed other symptoms including headaches and nausea.
- in both cases, the signs were not apparent until at least an hour after diving, and the signs subsequently endured for at least one more hour before oxygen was administered (I was dealing with the aftermath, not on the boat at the time), but in both cases the "mottling/rash" disappeared quite rapidly (within 30 minutes) after the diver was put on oxygen.
- it wasn't "localised" - as in - just on a bit of arm or shoulder - it was quite widespread in the affected area.
But - there are allergic reactions and so on that look similar. It's difficult to judge, but a reasonable guide to differentiating between the two is the colour of the rash - brighter colouration suggests it's closer to the surface of the skin, a darker purple sort of colour suggests that it's subcutaneous (under the skin) . Look for other symptoms (as in the aforementioned headache and nausea etc.) and a dead giveaway is the administration of oxygen. If symptoms persist, it's probably not DCS, if they disappear, there's a significant chance there is. If it's only on one specific location - such as the right shoulder, in one case I dealt with, then it's more likely to be an allergic or stress-related bruise or abrasion. Neural DCS can cause a diver to be "one-sided" due to bubbles in the brain or spinal chord, (similiar to a stroke) but from what I have seen, skin bends do not favour a side (the physiology is different), but are more broadly spread across the body.
That's not meant to be comprehensive, just what I know and what I've had to deal with. Both cases involved the diver being put into the chamber, both were easily resolved, both divers are still diving.
Cheers
C.