Yes - just log in to your FC account on the website and go to book a reward flight. You don't have to have enough miles to see the dates where there is availability. You just need to be logged in.
If you see a day available, then it has availability for reward/upgrade seats. If the day is blocked out (or no click button on it), then there are none.
Also, if you check back later regarding availability for a date that was previously unavailable VS may have released some seats. This is especially likely seven days prior to your desired flight.