How and when will I be able to find out if the flight will be cancelled?
Cancellation times vary a lot but 1-2 weeks before departure seems to be typical at the moment, so it's still too early. A good indicator is today's or yesterday's flight status of your itinerary. If the flights are cancelled or non existent, than chances are very high they will not be reinstated by April 19 and are likely to be cancelled in a couple of weeks.
Go to https://www.lufthansa.com/us/en/timetable-and-flight-status#/ and pop in your US first arrival (assuming it's operated by LH). If it shows "no flights" than chances are your reservation is going to be cancelled soon.
if Lufthansa cancels the flight I assume that I would get back the money I paid
Legally LH would be required to refund you the cash due to regulation EU 261. However, they are unlikely to do so and it will be very difficult to force them. Your best chance would be charge back through your credit card, so it's probably a good idea to contact your credit card provider and find out how this could work