You can usually quite easily distinguish them by their "looks":
Photo credits herpetofauna.at: Ch. Riegler, R. Kopeczky and me
P. rupestris (very common):

P. minimus (very small, sandy regions):

P. celerrimus (longest P. in Oman, mountains, notice striped tail)

P. gallagheri (usually prominent bright stripe on back, small distribution range)

P. carteri (very distinctive shape)

Send me your email-adress as privat message, then I can foward you a pdf of The herpetofauna of the Sultanate of Oman. Images are in bad resolution in this one, but I think still think better than nothing.
PS. it's a pity that you didn't buy the book before your trip

PPS. Oman is always worth a second trip! ..and a third. And i also heard a fourth
Cheers
Gerald