To know if an NFT is good you need to check some vitals, a good community, a social following with high engagement, and a business plan.
NFT projects with the highest amount of social and community engagement will have the greatest success. The community is a key part of the NFT. Without it you simply buy the art because you like the art and that is fine if you're a collector who wishes to own nice art.
However most NFT buyers are investors who are looking to invest and flip the NFT soon after purchasing, allowing them to profit and free up funds to invest in more NFT art. It's an addictive process of buying and selling which is what drives most people to buy NFT’s.
Everyone wants to buy into a good nft project that has a solid community and high levels of social engagement along with a solid business plan or roadmap. Done right it can be life changing giving huge sums of money in a short amount or time with minimal risk done badly you will be stuck holding an NFT that nobody wants, well not yet anyway.
In summary, look for NFT drops and projects with high levels of social activity. They may cost more than the less popular NFTs but it's a safer investment. All investments carry risk so don't invest more than you can afford to lose.