Do not train people to judge based on logic, colors, or fancy text. A fraudulent prompt can be extremely similar to a legitimate one, thus “do you look official to you” is an inadequate test.
I would split all the updates in two categories: the ones user actively searched for and the ones that ambushed them. If they went to the settings, app store, or the update menu in a certain app – let them update. If the message appeared while they were in the middle of a recipe, a game, a conversation, or a website – tell them to dismiss it as an advertisement. System messages can occasionally appear unexpectedly, but there is no harm in closing them and reopening the application normally.
Smartphones and tablets are convenient due to only a couple of update sources: the system itself and the app store. Computers have more update methods: some programs update via their own store while others have a “Check for Updates” button in the settings tab. For unexperienced users, it is better to stick to the global system and avoid everything that requires extra software: “download an updater”, “install a browser extension”, “cleanse drivers”, “scan the system with Java Safety Tool” etc.
I agree with @quantum_gadget282 that urgency is a useful giveaway, but I would not stop at closing the message. Repeated warnings usually mean the browser has notification permission for a junk site, or a questionable app is generating them. Otherwise your relative may follow the rule correctly and still call every afternoon when the same warning returns. Removing that notification permission or uninstalling the source is more work once, but much less work than evaluating each popup forever.
Automatic updates help, but they tend not to kill every prompt. The device could still ask you to reboot, provide more storage, enter a passcode, or approve an update you’ve already downloaded. The helpful comparison is that these prompts stay within a trusted UI context. It doesn’t take you out to a phone number, payment screen, chat UI, or unknown download.
I’d put a note in natural language near the device saying: “Only install updates from screens you’ve opened yourself.” That gives them a choice they can make without having to be able to parse icons, notification types or technical language