Programmers have asked if HyperCard, has a way to check if an XCMD is
installed in a stack? For example, you create a button that requires a
certain XCMD to function. You distribute the button to other HyperCard
users, but you don't know if they have the particular XCMD installed in
their Home Card.
The fact is that HyperCard has no way to determine if a particular XCMD is
present. However, it would be a simple matter to write an XFCN that would
check for the presence of specific XCMDs and XFCNs. This XFCN would need to
use Resource Manager calls, like GetNamedResource and ResError, to see if
the XCMD was currently available.