PL/I nowadays has two types of NIL values for pointers, the old, that has been around since the inception of PL/I on 24-bit systems, and is generated by the NULL() builtin, having a value of 'FF000000'bx, and the preferred new one, generated by the SYSNULL() builtin, having the value '00000000'bx.
However, you cannot compare pointers directly against these values. As Robert Sample said, you can only compare pointers to NULL() (or SYSNULL()) to determine if they are set (and never mix use of NULL() and SYSNULL() in the same program!). You can also compare a pointer to another pointer, but only for (in)equality.
However, PL/I has based storage and the UNION attribute. Both allow you to overlay a pointer with a "FIXED BIN (31)" and then compare the FIXED BIN (31) to whatever other numerical value you want, which should of course only be 0 or 4278190080 (aka 'FF000000'bx).
DCL P PTR;
DCL F FIXED BIN (31) BASED(ADDR(P));
DCL 1 * UNION,
2 P PTR,
2 F FIXED BIN (31);
And PL/I wouldn't be PL/I if there weren't a third way of doing things, and that's using the UNSPEC() builtin. This builtin only works on elementary variables (not on arrays or structures), and it will return the contents as a bit-string, but it is implementation dependent, i.e. it will return different values on z/OS and Windoze (yes, PL/I is available for the latter). Using something like
if unspec(myptr) = unspec(my_fixed_bin_31) /* or unspec(my_fixed_dec_7_comma_4) or unspec(my_char_4) */ then ...
you can compare anything with everything, without the compiler generating any messages, which is of course not something you should be doing, as a good programmer wants to get as many warnings as possible to inform him or her of as many things that need to be looked at!
I don't know what level of Enterprise PL/I you're using, but I've got a set of '*process' statements that will result in W-type messages for more situations than the standard IBM supplied compiler options give you, shout and I'll post them here, for EPLI up to Version 4.3, the latest version I can use.