I often see this question asked in groups. People post a pic of their pointed rat and wants to know if it is a Siamese or Himalayan?
The truth is that unless you know the rats pedigree and/or saw them as babies there is no way to know for sure.
While ideally a Siamese and a Himalayan should look different because of their popularity they boomed up in pet shops and bad breeding taking them both far from standards. So you can have a Siamese that looks more Himalayan and vice versa. Looks alone (after a molt) will NOT tell you.
I have seen so many people suggest you can tell because of how dark the points are, how the points look, or the color of the adult rat etc... but that is simply not true.
Ideally to standard, an ADULT Siamese will not be white but a darker shade and a Himalayan will be a clean crisp white with points but that is not usually the case. Some Siamese are very light.
You can tell by genetics....
Siamese, Himalayan, and PEW (pink eyed white/albino) are all on the C-Lotus.
You have the c recessive for albino and the c(h) recessive for Siamese.
Himalayan is the combining of the two cc(h)
aacc = Albino
aacc(h) = Himalayan
If you breed a Siamese to an albino you get all Himalayan.
Two Himalayans together will give you 1/2 Himalayan, 1/4 Siamese, 1/4 Albino.
Albino to Albino and you get all Albino.
Siamese to Siamese and you get all Siamese.
So if you know what the parents are you will often be able to tell. But it is important to note that they are recessive genes, meaning they can be carried and must come from both parents.
But if you do not know the genetics you can also tell by what they looked like as babies.
Siamese rats begin life colored. Seal points are a warm beige/brown color. Ideally as the standard they should be colored all over but many Siamese are marked. So they could be capped, hooded, etc...
Their color begins to fade as their points darken.
Himalayans start out life all white. They will then begin to develop points.
Both of them begin to develop points when they first molt around 6 weeks old. In truth it is alittle bit different for every rat. Sometimes points will begin to show alittle earlier or later. But in general around 6 weeks it begins.
This is one of my favorite pictures of my Siamese boy Dean. As a baby he was so dark! His color began to lighten and fade away. He was still in the middle of his molt in the final pictures.
Ideally to standard they should NOT be marked, meaning it is a self rat (all one color) as opposed to marked meaning it has markings ie hooded, blaze, etc....
Sadly most out there are marked. This often muddies up their standards and gives sub-par color points. Their nose may not be as dark, no or light butt points, and white feet.
IMO good breeders will want to breed towards standards and improve their Siamese/Himalayan lines by working towards unmarked lines. It is a process! Breeding to black selfs can help to improve it!
But marked Siamese still are lovely pet rats!
So if you came upon your rat past their molting age with points already and do not know what the parents were....
I get the need to try and form some type of clarification on the variety for when you describe it to people. The truth is as long as you are not breeding the rat it doesn't matter what you call it. Pick one!