If you need to test subscriptions or recurring payment IPN's you can easily do so by creating your own HTML form and setting the action to your IPN listener URL.
Add hidden fields to the form that match the fields you would expect to get from PayPal, which you can see by doing an actual transaction on the sandbox to get that first IPN, and then duplicate it in your test form.
This way, you can submit the form directly to your IPN listener and actually see the result on the screen. This is also useful for troubleshooting.
The only thing you need to consider is that when testing this way the IPN will not verify because the original data did not come from PayPal. You could easily handle this in your code for testing purposes.
Once you have everything working the way you want like this you can rest assured it will work the same way for actual IPN's received from PayPal.