By just looking at the picture you posted its almost impossible to tell. I just saw other recent pictures from Marana (all shot by busy Sam Chui) where the scene above is more in close-up.
Otherwise the easiest thing to tell a 744 apart from a 743 are the winglets. The 744 has differently looking engines of a newer generation (although a few late-built 743s from Thai and Varig got the 744-type GE
-engines). Another distinction is the wing-root which is more aerodynamic on the 744 (again, a few late-built 743s, namely from Egyptair and Sabena, have that feature, too). A fourth point is the antenna in the middle on the back of the fuselage which only the older 747s (741, 742, 743 and SP
) have. To make it difficult again, some of the newest 744s, built after 2000, again have an antenna blade in a similar position.