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From television to cosmetics and everything in between, Amazon is promising to have “millions of great deals” during its 10th Prime Day sales event. While some Prime Deals are already live, the main event begins at 3 a.m. on Tuesday and lasts 48 hours.

For shoppers, the key is telling whether a deal is truly a deal. Thankfully, there are websites and apps that can help with that.

Before clicking “buy now” or adding an item to your cart, you should first review its price history. The website camelcamelcamel.com allows you to input the link to the product and see the price history for the product.

There is also an app called Keepa which allows you to browse Amazon’s website within its app, and it will automatically insert the price history into the listings, so you know if today’s deal will cost you more or the same as last week’s price.

Both camelcamelcamel and Keepa show your price histories, similar to how a stock tracking website tracks a stock’s price. Keep in mind, not all products will have a price history.

Another tip, check the price of items with other retailers as well. Prime Day may be Amazon’s shopping holiday, but it doesn’t mean other big retailers are not trying to take advantage of the interest in it by offering better deals on Prime Day items.

Walmart in fact will begin its Walmart+ Week on Wednesday and continue it through July 23.

New this year are invite-only deals, which limit who can buy an item and with those deals, instead of an “add to cart” button, you’ll see a button which you can click to request and invitation to purchase the item. “You’ll be notified via email if you are invited to take advantage of the deal.

Prime Day is also known for its “lightning deals” which are deals that are available during limited windows of time. To not miss those, for items marked as upcoming lightning deals, add them to your cart without checking out. Once the lightning deal starts, refresh the app or browser until you see the deal price and check out.

Also, Amazon Prime Members can set up alerts on the Prime Day page, which will alert you when deals you’re waiting for go live.