the sonatas were written with the harpsichord in mind and therefore have no marked dynamics.
This answer is relatively true, but not 100% accurate. Scarlatti DID have the option of varying dynamic colours...and was well aware of the possibilites of the fortepiano (Maria Barbara had several fortepianos where Scarlatti worked for her in Spain). Even so, it is quite easy to get a variety of dynamic contrasts on the HARPSICHORD by using the two manuals (Haydn exploits this in some of his sonatas). As for the original question...well, make sure you notice the register changes, and the use of voicing in Scarlatti's sonatas. THIS is exactly how he compensated for the harpsichord's lack of dynamic contrast as compared to the fortepiano.
In essence, I wouldn't worry so much about dynamic change (piano to forte, etc.). I would concentrate more on the colour of the sound. When there is a significant change in register, or an obvious change in texture...change the colour of your playing (using pedals or not...this is where you can be creative). Too much emphasis is placed on accurate performance practice, when in all reality, we have no idea how these things sounded in Scarlatti's time. Of course this debate has raged on in scholarly music journals for decades...but it is no coincidence that they still haven't reached a conclusion that everyone agrees with!

Also, remember that you are playing his works on the piano, not the harpsichord...so I wouldn't try to immitate the harpsichord...make them your own and bring them into the 21st century...after all, that's what Liszt did in his own century with music from the masters of the past...or...Busoni with Bach.
Good luck...Scarlatti is amazing, and quite a revelation to play!
- Andrew
P.S. If you are interested in a VERY good analysis of the sonatas, see Ralph Kirkpatrick's book entitled "Scarlatti". It's an amazing piece of literature, and probably remains the best-known (for a reason) book on Scarlatti's life and music. You could also try Malcolm Boyd's book...but he tries to stay away from the sonatas in his writing because Kirkpatrick's earlier effort was so complete and well done (Boyd studied with Kirk.).