As mentioned above - stars on the obverse and mintmarks are the main obvious things. See the other threads on differentiating a 1916 from a 1917 Type 1.
The dateless ones are pre 1925 about 80% of the time. Sometimes, under magnification you can make out the last number of the date which might enable you to make a determination. But many dateless (most) are so obliterated that you'll never know.
Do not try any cleaning with any chemicals to aid this, you'll only destroy any value the coin may have.
Good luck and welcome to the sickness.