You can only go by phenotype unless you know without a doubt the genotype of your rats. If your rats come from unknown backgrounds, you have no way of knowing their true genotype. Even from known backgrounds it can sometimes be tricky unless they have consisently been "color bred" for a number of generations and given consistent colors that clearly meet one of the standards.
Siamese = ch/ch
Himalayan = ch/c
Albino (aka PEW) = c/c
Siamese x Siamese = all Siamese
Siamese x Himalayan = Siamese and Himalayan
Siamese x Albino = all Himalayan
Himalayan x Himalayan = Siamese, Himalayan, Albino
Himalayan x Albino = Himalayan and Albino
Albino x Albino = all Albino
A good Siamese will have rich, dark points and good shading.
A good Himalayan will have rich, dark points, but no shading.
A good PEW is going to be a clean clear white all over.
A light Siamese can resemble a Himalayan. A dark Himalayan can resemble a light Siamese. This is why you can't truly know what you have genetically if you don't know the backgrounds, and why you need consistent results before you can say with little doubt what your animals are genetically. Siamese bred to Siamese should consistently produce Siamese, and if selected correctly will produce better Siamese over time. If your "Siamese" is bred to "Siamese" and you consistently get mixed results, you are possibly breeding genetic Siamese and Himalayans together.
A GOOD Siamese is going to be dark as a baby. You may not necessarily see points, per say, but you will see good shading that becomes darker where the points should be. The points on a Siamese are going to become most apparent after 6 weeks of age when the baby fur is shed and the adult fur comes in. A Himalayan is going to start out looking white(ish). The points will start coming in at about 6 weeks when the baby fur sheds and the adult fur comes in. However, a very light Siamese can resemble a dark Himalayan, even as a baby, and vice versa. This especially true of UK stock bred for Burmese (in England breeders select for lighter, poorly shaded Siamese and darker Himalayans when breeding for Burmese with the idea that it makes the points better).