Comments are not searchable using our internal search; they're meant to be ephemeral chatter that is normally removed after the results of a brief conversation are moved into the parent post, so it doesn't make much sense for us to make them searchable.
There are, however, some comments that stick around because they have lasting value, but don't really belong in the post itself. For that, yes, you can use Google or other search engines if you've got an idea of what the comment contains. Just use the
site: modifier (for Google) and you can probably turn them up.
The best thing to do in order to make content easy to discover is to make sure the title is as descriptive of the question as possible, and that the question is well explained. In our experience, Google and other search engines are quite good at connecting folks with the best subset of questions in search results. Tagging is also important here, as the system quite often incorporates the most heavily used tag attached to a question in the title.
For instances where you've got something where folks might search a number of ways, you'll probably end up with people asking duplicate questions because nothing turned up for them in their particular query. When this happens, just mark the question as a duplicate of the best version of it that you have, and it becomes a sort of 'stub' to lead people to a canonical answer on the topic.
In short, don't worry too much about SEO beyond clear titles - a feature of our engine is that it's quite good at getting that right.