Use your browser (or other TLS client) to view the certificate after connecting, or just look at the certificate file with a tool that will parse it for you (such as the certificate viewer built into Windows). Look in particular for the "Common Name" and "Subject Alternate Names" (SAN) fields, which will list the domain name(s) for which the certificate is valid. If either one starts with
*. - as in,
*.yourdomain.com - that indicates that subdomains are supported (typically such certificates will also list the root domain,
yourdomain.com, as one of the entries in one of those fields). The
* is called a "wildcard", and a cert that uses one in the CN/SAN fields is called a "wildcard certificate". Wildcard certs are sometimes more expensive.
Note that a cert can cover many domains (and/or subdomains of one or more particular domains) explicitly, without using a wildcard. Certs that cover multiple domains are often more expensive, since it's essentially multiple certs in one.
I'd be remiss to not mention: you can get a cert issued without paying anybody at all. LetsEncrypt is a well-known free certificate authority, trusted by all major browsers, and they will not only issue as many certs as you need for free, they provide tools to automate the renewal of these certs (their expiration is short, but automatic renewing is easy on most platforms).