How can I identify LEGO stickers? What steps can I take to identify LEGO stickers/sticker sheets and which sets they belong to?
Using the full sticker sheet for #60150 Pizza Van set as an example, there are a few options.


If you have the full sticker sheet, you might be able to find a part number for the sticker sheet (in this case 29583/6174606), it can usually be found at the bottom and/or near 'LEGO GROUP'. Use either Bricklink sticker search or Google to find the original set. Older LEGO sticker sheets might only use the set ID.
A lot of sets, particularly those released in the last two decades and particularly CITY sets will use the set ID on vehicle number plates. Usually for number plates, the first two letters will be the initials of the set designer and the following number would represent the set ID. They can also be found in more creative places such as a tail number for a helicopter.
Google Image Search has a function called 'Search by Image', this can be found be pressing the camera icon in the Google Images search field. Take a picture of the sticker sheet, or a single identifiable sticker. Ensure that the photo is in focus, well lit and from directly above and upload.




Google will try to find keywords to match the image uploaded, this isn't always helpful so you may need to replace the keywords with 'LEGO', 'Sticker' and if possible, the theme that the set might belong to.




If everything else fails, you can use the registered trademark year to narrow down the year it was produced/released. In this case, the release date must be during or after 2017. It's possible that a sticker sheet could be reprinted in following years to correct a flaw in the design and that the year could change.




Tips


The first sets to have stickers were released from 1971 onwards.
[Source]
Steps 2 and 3 can also be used to identify some printed parts.
If you find a sticker attached to a rare part or a part produced in a rare colour, it might be worth finding that part on Bricklink and finding the sets that it is used in.