If you want to check this inside the some hook, which runs after plugins_loaded hook, it is very simple. Just check some plugin specific constant or function exists or not.
For example, to check WooCommerce we will need to check any WOO constant is defined or not. Or it is enough to check if class WooCommerce exists or not.
But what if you want it to check it before the plugins loaded? In this case you will not be able to check some constant,function, class of the plugin is defined or not, because they are not loaded yet. So we will need some additional trick for that. We can check $wpdb->options table and get the list of active plugins, process it and get the results. Let’s see how it works.