The first step to identify if a card is potentially worth anything substantial is to check its type and rarity. In other words, how likely the card is to come across from any given booster pack or special event, and if it has any special characteristics, such as card material or Pokemon types.
There are a few indicators of rarity on any given card, though they’re quite subject to change depending on when said card was printed. A marker near the bottom of the card or near its name will indicate rarity: a circle means the card is common, a diamond means it’s less common, and stars mean it’s rare. More stars or with combinations of letters or symbols mean extra rare, including if those symbols are in the name or elsewhere on the card. Other characteristics that can up the rarity include: a higher printed number than there should be in a given printed set (e.g., 66/65); holographic artwork or reverse holographic, in which everything but the artwork is holographic; artwork that takes up the full card; artwork wherein the creature doesn’t cast a shadow; and any shining characteristics, not to be confused with holographic.
There are also special types of cards, like from the aforementioned special events that sometimes award cards to tournament winners. For example, one of the most coveted cards, the Trophy Pikachu Trainer Card, was only given out to competition winners in Japan and is so rare that it is considered priceless due to lack of sellers. And a Pikachu Illustrator card, awarded for a Pokemon award competition, allegedly sold for a cool $90,000 USD.
While rarity isn’t the only factor in appraising value, it’s certainly a major factor: Some of the highest-selling cards worth tens of thousands of dollars, or considered “priceless” from lack of supply, are only so because of small rarities like misprints or typos.
As for the more common cards that don’t match any of these characteristics, the consensus advice online is to sell those in bulk. Though the individual cards may only be worth a few dollars at most, a complete collection of them can likely fetch a little higher of a price. That’s not a terrible idea for someone wanting to get rid of a bunch of common cards and turn a profit.