Looking at $ENV{SHELL} is probably the best we can do --- other programs seem to do it the same way (and therefore do it wrong):

$ env SHELL=/bin/sh ssh-agent
SSH_AUTH_SOCK=/tmp/ssh-XXN54U4g/agent.3950; export SSH_AUTH_SOCK;
SSH_AGENT_PID=3951; export SSH_AGENT_PID;
echo Agent pid 3951;
[download]

$ env SHELL=/bin/tcsh ssh-agent
setenv SSH_AUTH_SOCK /tmp/ssh-XXPC7hgb/agent.3948;
setenv SSH_AGENT_PID 3949;
echo Agent pid 3949;
[download]

Similar output for /usr/X11/bin/resize