I am really not sure which of these is meant here, and I think it can probably mean either. They are not the same thing, of course; a speckled hen is an ordinary domestic hen (Gallus gallus) with speckled plumage, whereas guinea fowl (or guineafowl) are various species of the sub-family Numididae, somewhat larger than ordinary hens (chickens), but also with speckled plumage.

The definition DLyons quotes is the only one I can find for "gallina sarada". It refers to "plumas de diversos colores, grises, blancas, negras, etc." But the translation of "sarado" (from El Salvador) offered on this page is "manchado", so that is apparently what is meant.
http://www.tubabel.com/definicion/30956-sarado-da

A "gallina manchada" is what we call a "speckled hen" in English. Here, for example, is a picture of a "gallina manchada de Sussex":
http://es.dreamstime.com/fotos-de-archivo-libres-de-regal-ia...
And here is a "speckled Sussex chicken":
http://en.wikipedia.org/wiki/File:Speckled_Sussex_Chicken.JP...

"Sarada" is a variant of "zarada" (or vice versa); the expression "gallina zarada" is used, for example, in Balún Canán, by Rosario Castellanos:
"Entonces fue necesario conseguir el huevo de una gallina zarada y con él fue tocando toda la superficie del cuerpo de Matilde mientras rezaba un padrenuestro".
http://books.google.es/books?id=I7lNiFbj-0oC&pg=PA164&lpg=PA...

In the following page the mosquito that carries the dengue virus is said to be "fácilmente reconocible por su aspecto “pintito” como “gallina zarada” o como una cebra"; the accompanying photo shows that it is speckled:
http://vet-pro-florafauna.blogspot.com/2009_11_01_archive.ht...

The element of superstition surrounding this bird, reflected in the Rosario Castellanos quotation, is also evident here, where someone asks:
"¿Es verdad que comer gallina zarada o gallina guinea que es lo mismo, da "Bienteveo" o Vitiligo?"
http://es.answers.yahoo.com/question/index?qid=2008040422392...

Vitiligo is depigmentation of the skin, so the idea is that eating a speckled bird makes you "speckled", producing light patches on your skin.

But also, this last quotation says that "gallina zarada (sarada)" is the same thing as "gallina guinea", so that suggests the possibility that it might refer to a guinea fowl.

Here are some images of speckled hens and guinea fowl:
http://www.google.es/search?tbm=isch&hl=es&source=hp&biw=991...
http://www.google.es/search?hl=es&biw=991&bih=576&gbv=2&tbm=...

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Note added at 4 days (2012-02-17 08:56:31 GMT) Post-grading
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Thanks. Since posting this, I've mentioned it to my (Spanish) wife, who grew up in the country, and she says speckled hens are thought to be good layers. I don't know if there's any truth in this, but I do think "sarada" is probably there for a reason and should be translated.

Guinea fowls are indeed a completely different bird. I think speckled hen is more likely to be what the author means, but see what he says.