You monitor the cached objects to see how often they expire and if they are "nuked".
varnishstat -1 | grep "n_expired\|n_lru_nuked" MAIN.n_expired 42220159 . Number of expired objects MAIN.n_lru_nuked 264005 . Number of LRU nuked objects
The one to watch here is
n_lru_nuked, if the rate is increasing (the rate, not only the number) your cache is pushing out objects faster and faster because of lack of space. You need to increase the cache size.
n_expired is more up to your application. A longer time to live will decrease this number but on the other hand not renew the objects as often. Also the cache might require more size.