One way would be to make an iptables rule that just detects traffic going to or from the SSL-enabled port (and possibly outright accepts it ... whatever). Then you could see how many packets and how much traffic is hitting that rule by checking the output of "iptables -nvL".
A more direct route might be to look in the apache logfiles, or the server-status page.
SSL operations aren't handled by the kernel, they're handled by the SSL library (typically openssl), and typically hooked by an app like Apache. The kernel's capacity to do various things is very unlikely to be the limiting factor in a real situation, and openssl's pretty fast at what it does too. If you're having problems with performance on a real website, you'd likely be better off optimizing the apache config, the database config, the actual database schema and queries hitting it, or improving interpreter performance for things like php (via opcode caching, fcgi).