How do you determine what coin is worth a damn versus a shit coin? Because I think it is pure luck if someone just happens to start buying the damn things based off my observations of this currency.
Well... First thing's first... You should generally know what the coins in the top 20 or so do and what is unique about them. Not ALL of them, but a good portion of them. You should know the difference between PoS, PoW, Oracles, the difference between coins and tokens, the difference between blockchain and digital currency, and the difference between centralized coins vs decentralized coins. If you don't, you have homework to do.
You can start with finding out the difference between Bitcoin and Ethereum, then investigate what Ripple does differently, then Cardano, then Chainlink, then Uniswap, then Monero, and yes, even Tether. That should give you a good basic overview on how things work differently from each other and the vast amount of functionality that is out there. Then you can go to the next step.
For new coins, the first thing to check is what they bring to the table. Is it something that other coins already do, does it do the same thing in a different way, is it a blatant copy, or is it something completely new? If it doesn't do anything original, it's a shitcoin. If it uses the name of another coin or asset and looks to be using that as a way to attract users, it's a shitcoin. If it advertises something, but is functionally incapable of doing that thing, and there are no signs of a roadmap or everything keeps getting delayed with no progress, it's a shitcoin.
For existing coins, you can do the same thing, and see if there is a coin with a higher market cap that already exists... Additionally, you can verify if the team behind the coin is still active, and how long they have been active.
Above that, you can check on which exchanges they are available. If they are on the biggest ones like Binance, there is a smaller chance of them being a shitcoin, simply because exchanges like Binance have a steep entry for the ones wanting to list their coin, and they have no issues removing coins either. There's a reason coins that get (announced to be) listed on Coinbase generally tend to increase tremendously in price.
The coins that are highly likely not shitcoins are the ones that focus on DeFi or focus on being oracles. But there are always exceptions. Yes, this world is complicated and it takes time to learn and understand. If you click a coin at Coingecko for example, scroll down and read its description. It should give you an idea of what each coin does. If its description is only about its price, it's most likely a shitcoin.
But Bitcoin is just open source code running over a network. Anyone can copy, fork it. There are several popular forks of it like Litecoin. They just change some values in the code (like block size) and voila. There's nothing unique about it except being THE FIRST.
This is a perfect example of what to look for. Litecoin might be a fork of Bitcoin, but, the difference is that Litecoin uses Scrypt algorithm rather than SHA-256 algorithm.
If you ask why they changed the algorithm, it's a good question. It was supposed to solve the problem of Bitcoin becoming too centralized. ASICs were being developed for SHA-256, meaning normal people could no longer compete with mining against the ones that were mining with ASICs. Mining was only viable with ASICs, and using a graphics card would mean running at a loss. Litecoin was created to solve that problem, to be ASIC-resistant, so that people could mine with their normal hardware at home.
Ultimately, ASICs were created for Litecoin as well, sort of defeating its initial fork purpose. Does that mean that Litecoin is now irrelevant...? Well, not really.
Two reasons... Just like the USD doesn't make the EURO irrelevant, Bitcoin does not make Litecoin irrelevant. Just like separate country/regional currencies can exist, Bitcoin and Litecoin have become separate currencies within the crypto space. Moreover, Litecoin's team is still active, and they are working to implement private transactions, something that Monero and ZCash are already doing, but in a completely different way from each other. Only time will prove which implementation is better, but chances are that they will simply co-exist and interact with each other through Oracles in the future.