There was just a really long post on this on the main forum. Bottom line is you can't tell the difference by looking, but you need to run color tests.
Cremello is RED based, with 2 cream genes (double dilute), and may or may not have other genes such as agouti (which causes bay or buckskin in combination with other genes, but has no effect on red).
Perlino is BLACK based, with 2 cream genes and agouti
Smokey cream is BLACK based, 2 cream genes, without the agouti
So to tell the difference you would need to test for red/black, and agouti. Sometimes (as was the case on the other thread) the horse might also look like palomino, so in that case, you would also need to test for cream to see if it had 1 or 2 cream genes.
Here is link to other post:
Forum post on cremello/perlino