How can I determine whether xyz.example.com uses a wildcard A record?

You can literally query "*.example.com" and find out if there is a wildcard, but it is impossible to tell the difference between these two zones:

xyz.example.com.  IN A  1.2.3.4
*.example.com.    IN A  1.2.3.4

and

*.example.com.    IN A  1.2.3.4

i.e., you can't find out whether you're being answered by a wildcard for a given query, only that a wildcard exists.

I haven't found any Web-accessible looking glasses that support it yet, as they seem to think it's invalid input, but raw dig (or even nslookup on Windows) works like a charm:

c:\Some\User> nslookup
> *.my-test-domain.com
Server:  Wireless_Broadband_Router.home
Address:  192.168.1.1

Non-authoritative answer:
Name:    *.my-test-domain-is-not-a-real-domain.com
Address:  1.2.3.4

Or with dig:

# dig +short '*.not-a-real-domain.com'
1.2.3.4

In response to BMDan's good answer:

houdini@linode:~/ > dig '*.blogspot.com' +noall +answer
*.blogspot.com.         3352    IN      CNAME   blogspot.l.google.com.
blogspot.l.google.com.  52      IN      A       72.14.204.191
houdini@linode:~/ > ^blogspot.com^undermyhat.org
dig '*.undermyhat.org' +noall +answer
*.undermyhat.org.       14400   IN      A       83.222.226.157