Background. To learn more about how to mitigate the BEAST attack, I recommend reading the following two blog posts: Rizzo/Duong BEAST Countermeasures and Mitigating the BEAST attack on TLS. You'll quickly learn that there are no really great solutions.
What to scan for. Right now, the only known server-side mitigation is for the server to force use of RC4 by refusing to accept any other ciphersuite. A similar variation is to place the RC4 ciphersuite as the highest preference; this is almost as safe, as it ensures that any client which supports RC4 will use RC4.
Thus, when you scan a SSL server, if the server prioritizes any block cipher based ciphersuite higher the RC4 ciphersuites, then it can be considered vulnerable to the BEAST attack. If all the ciphersuites it accepts use RC4 (or if the RC4 ciphersuites are prioritized before any block cipher ciphersuite, except for TLS 1.2-only ciphersuites), it is probably safe from the BEAST attack.
Determining the server's priorities on ciphersuites can be tricky, and may require multiple connections to the server with a different set of ciphersuites each time. However, in this case one simple method may be to have your client connect with a list of ciphersuites that starts with all of the (non-TLS-1.2-only) block cipher suites, and then end with some RC4 ciphersuites. If the server picks any block cipher ciphersuite, then the server is probably vulnerable to the BEAST attack.
Ideally, the server would support TLS 1.1 or higher. If both the client and the server support TLS 1.1, then the BEAST attack becomes much harder (it requires a man-in-the-middle attack). Therefore, if the server doesn't support TLS 1.1, you might want to output a warning/advisory suggesting that the server admin upgrade to provide TLS 1.1 support. However, apparently supporting TLS 1.1 on both endpoints is not an absolute guarantee of security, due to the possibility of downgrade attacks if a man-in-the-middle is present, so I would suggest listing this merely as a warning/advisory, rather than a critical failure.
More on SSL server scanning. You might want to consider using the SSL Labs service for scanning a SSL server. It is a great way to test for many possible configuration vulnerabilities. It will also test whether the server is vulnerable to the BEAST attack, but it will check for many other common and serious problems as well -- which is probably more useful than just looking for the BEAST problem.