If the liquid is cloudy or turned dark brown then it's probably not any good. PG turns cloudy as it goes bad, nicotine and flavorings gradually turn brown and change to a sort of sludge like mixture. But if it's still clear, viscus, and normal colored it should be just fine.
PG and nicotine base has a shelf life of about 2 years if kept in the dark at room temperature. Vegetable glycerin has a shelf life of 4-6 years. The shelf life of flavorings can vary wildly depending on the flavoring and what it's mixed in. Most alcohol flavorings can last 3-4 years, PG based flavorings 1-2 years. Artificial flavors will last longer than natural flavors.