There's the GIMME_V macro which will tell whether you are in scalar or array context; there's the PL_op macro to get the current OP, which should be 166 for ego itself (method). A COP structure has a member op_next.

    printf("current: %d\n", PL_op->op_type);
    printf("next: %d\n", PL_op->op_next->op_type);
        
    if(GIMME_V) {
        printf("array context\n");
    } else {
        printf("scalar context\n");
    }
[download]

perl -le 'use blib; use Alter qw(ego); $f = ego($a, { 1, 2 });'
current: 166  # OP_METHOD
next: 6       # OP_GVSV
scalar context

perl -le 'use blib; use Alter qw(ego); $f = {ego($a, { 1, 2 })}'
current: 166 # OP_METHOD
next: 144    # OP_ANONLIST   huh?
array context

perl -le 'use blib; use Alter qw(ego); $f = [ego($a, { 1, 2 })]'
current: 166 # OP_METHOD
next: 143    # OP_LSLICE     huh?
array context
Assertion *strp failed: file "av.c", line 392 at -e line 1.  wtf???
[download]

But the next OP could also be something entirely different, depending on position...

perl -le 'use blib; use Alter qw(ego); $f = {ego($a, { 1, 2 }),1}'
current: 166 # OP_METHOD
next: 5      # OP_CONST
array context

perl -le 'use blib; use Alter qw(ego); $f = 1,ego($a, { 1, 2 }),1'
current: 166 # OP_METHOD
next: 141    # OP_JOIN
array context
[download]

Anyways, I hope that helps and gets you on the way :-)

<update>

For your examples...

perl -le 'use blib; use Alter qw(ego); ego($a, { 1, 2 })->{"foo"} = "b
+ar"'
current: 166 # OP_METHOD
next: 134    # OP_EXISTS
scalar context
Segmentation fault

perl -le 'use blib; use Alter qw(ego); @{ ego( $a, { 1, 2 }) } = (1,2,
+3)'
current: 166 # OP_METHOD
next: 178    # OP_ENTER    hmm?
scalar context
Segmentation fault

perl -le 'use blib; use Alter qw(ego); ego( $a, { 1, 2 })->[0] = (1,2,
+3)'
current: 166 # OP_METHOD
next: 125    # OP_QUOTEMETA  interesting...
scalar context
Segmentation fault
[download]

I guess the segfaults are quite normal, since ego() isn't type aware yet... but maybe overload() or WANT might suit you better, as ikegami and BrowserUk pointed out, since relying on how perl organizes its opcode chain is - um, well - scary?

</update>

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                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}