Is there any way to check whether the user has an SEO plugin installed, so as if they do not I can insert my metas etc.
What you are going to need to do is create a list of the top SEO plugins and themes and then document how each implements meta and develop a strategy for each. And/or maybe you can ask your users upon activation if they are using an SEO plugin much like Akismet asks users to enter an API key.
You can check for specific plugin(s), for example with
But there is no way to check for SEO plugin in general, at least not in any kind of reliable way.