Are my cookies cooked? I've just recently started baking and I'm finding it quite enjoyable. I am now changing some of the recipes a little bit to suit my tastes better.

A problem i am having is that the ingredient swaps often cause the texture to change slightly so some may need more or less baking time than before to get to a good consistency.

Is there an easy way to check if a cookie/biscuit is ready to be put on the cooling rack? So far i've been tapping them with a fork to feel how firm they are.
There isn't a universal way to tell. It's going to depend on the type of cookie. For chocolate chip cookies, I watch for a slight amount of browning around the edges. If you're using a chocolate flavored dough, that won't work because the dough is already dark.  So you'd have to watch for changes in the surface, such as cracks or dullness. The bottom line is that you're going to need to read the recipe and then experiment with it until you learn how to get it right.

As a general rule, the cookies will still be very soft and appear slightly underdone when you remove them from the oven. Leave them on the baking sheet for at least five minutes before transferring to a cooling rack. They'll set up during this time due to residual heat.