How would I identify the grounded (neutral) conductor when using URD cable as an underground feeder?
The AA-1350 here is legal, but tricky

URD cable is also known as type USE (Underground Service Entrance) cable, not to be confused with type SE style U (Service Entrance - Unarmored) cable. As a result, NEC 310.106(B) permits AA-1350 alloy to be used for it:


  (B) Conductor Material. Conductors in this article shall be of
  aluminum, copper-clad aluminum, or copper unless otherwise
  specified.
  
  Solid aluminum conductors 8, 10, and 12 AWG shall be
  made of an AA-8000 series electrical grade aluminum alloy
  conductor material. Stranded aluminum conductors 8 AWG
  through 1000 kcmil marked as Type RHH, RHW, XHHW, THW, THHW, THWN, THHN, service-entrance Type SE
  Style U, and SE Style R shall be made of an AA-8000 series electrical grade aluminum alloy conductor material.


Since it's AA-1350, though, you need to be picky about your terminations.  Torquing the lugs at each end to manufacturer specifications with a calibrated torque wrench is essential here, and so is the proper use of anti-oxidant grease.

The neutral should be the striped wire

It appears that one of the wires in the cable should have a striped jacket.  If so, that is your neutral wire -- there is no need for further marking.

If such a marking is absent, then since you are dealing with 4AWG or larger wire, you can mark the neutral wire at each end using wraps of white electrical tape (phase tape).  (4AWG or larger wire puts you under 200.6(B) instead of 200.6(A) in the NEC, by the way.)

As to fitting it down the pipe...

Type URD/USE cables, as a rule, lack an outer jacket.  As a result, they are governed by provisions in Chapter 9, Note 9, that allow you to use the sum of the individual conductor areas instead of the cable area for fill calculations:


  (9) A multiconductor cable, optical fiber cable, or flexible
  cord of two or more conductors shall be treated as a
  single conductor for calculating percentage conduit or
  tubing fill area. For cables that have elliptical cross
  sections, the cross-sectional area calculation shall be
  based on using the major diameter of the ellipse as a
  circle diameter. Assemblies of single insulated conductors without an overall covering shall not be considered
  a cable when determining conduit or tubing fill area.
  The conduit or tubing fill for the assemblies shall be
  calculated based upon the individual conductors.


As a result of this, the cable you have comes up to 77mm2*3 for the 2AWG USE-2 insulated conductors and 21mm2 for the bare 4AWG ground, or 252mm2.  Given that the smallest 1.5" conduit in terms of usable area (Schedule 80 PVC) can accommodate 442mm2 of fill, you should have plenty of space for this pull.