TCGPlayer is best known for its own sales hub- the TCGPlayer Marketplace. With current market listings, aswell as previously sold examples, it can be an excellent place to check pricing on cards.
The bonus of different collection lists being available means you can track your card, or collections value over time with completely custom lists!

• After downloading the TCGPlayer app, go to Collection.
• Create a new list. You can select add with the scanner, or add manually.
• If scanning, place the card on a dark background and align the card with the guidelines on your screen.

-Once it scans, ensure that the set icon in the bottom left corner, or middle right side is accurate, aswell as the card number in the bottom left, or right corner.
-Change the condition to accurately match your card.

(You may need to note the card number incase there are multiple of that pokemon, or versions of the card in a set.)

ebay is a market leader in online sales of every sort! Launched in 1995, its the first place many think of for sales, and that makes it the perfect place to check pricing on your cards using recently sold listings.

• Enter the Pokemon name, found at the top left corner of the card, along with the set number of the card, typically found in the bottom left corner for modern cards, or bottom right corner for vintage. (Example: Charizard 146/144)
• Filter your search to show "Sold Items"//"Completed Items", and sort by "Ended Recently".
• Use photos and item descriptions to ensure you're using sold listings accurate to your item to price it.
(Raw cards won't sell for slab prices- and damaged cards won't sell for near mint prices. Being sure the listings are accurate will save you a lot of time and offers.)
• Add up the 3 most accurate recently sold listings, then divide by 3 for an average price.