Please excuse my lack of knowledge, but I have a lot of coins from the same year's and some have mint Mark's and some don't. What's the best way to find out what they could be or not be in value?
To understand value, many things come into play. Demand, rarity, and condition are big contributors. Buy a Red Book , not so much for the value of a coin, but for its in-depth amount of information about all US coin denominations and each issue within the type of coin needing information. Welcome to CoinTalk and Coin Collecting.
For the US One Cent (Penny) without a Mint Mark.. They were minted at the Philadelphia Mint. They did not use a Mint Mark until 2017. First and maybe the last time they will use it. D is for Denver Mint and S is for San Francisco Mint.
As paddyman 98 said, P on any US coin is Philadelphia, D is Denver, S is San Francisco, W is West Point.
All business strikes that you will find in your change from the 60's, 70's & 80's that have no mint mark are struck at Philadelphia. They did not utilize a mint mark. Best resource in the world to figure out if you have anything of value is the RS Yeoman's Redbook of United States Coins. Very available book. Been published for years. You can find it most anywhere quite affordably.
Welcome to CT, @Nosaj Yeah, the (Yeoman's) Red Book is the reference of choice...& while a new one is published each year, you don't have to have the latest...a 5 or 6 yr old copy will suffice & you can find it readily & inexpensive @ places like Half Price Books or Amazon. As for values, disregard the those listed in the book, as they are outdated the moment the book is published. But you can interpolate that info to see relative differences in values for rarities & varieties, as well as checking the "sold" coins on sites such as eBay & Heritage Auctions (don't be concerned with "asking price" - you want to know what they sold for; the more recent the data, the more accurate it is likely to be). Good luck hunting/collecting!
Welcome to CT. If you posted here, you are online. Go to ebay and see what they are selling for. You'll get a range, not just one value.