Most of the content is unofficial, but there are ways to differentiate them
There are "four" types of products published in the Guild: unofficial content, AL-legal content, content released by Wizards of the Coast, and "DMsGuild Adepts" products.
Most of the content published in DMsGuild is unofficial content (aka, considered homebrew or third-party). To identify if a product is unofficial content, look at the Product Information sidebar on the product's page right side: it will contain the "Community Content" logo, indicating it was published by a member of the community. Another important information, is that the Publisher of the content (bottom of the Product Information sidebar) will always be the Dungeon Masters Guild.
Community Content logo.
However, there's a chance that such a product may be AL-legal. But there are ways to find out if it is AL-legal or not.
AL-legal content appears under the "Newest D&D Adventurers League Titles" slide in the DMsGuild front page. They are usually identifiable by their name (usually starting with the CCC or DDAL prefix), and they are always categorized with the "D&D Adventurers League" tag under the Browse Categories sidebar (left side) of the website.
The vast majority of the products under this label are adventures for DMs to run under the guidelines of Adventurer's League.
They are usually released by members of the Community (and will include the Community Content logo above), but sometimes, Wizards of the Coast releases material for use with AL in the Guild too.
Content by Wizards of the Coast
Wizards of the Coast also publishes content in the DMsGuild. To find if something is published by Wizards, look at the Product Information sidebar: it will include Wizards logo as the Publisher.
However, among the material published by WotC for D&D 5E, there is both unofficial and official content, considering 5E rules. Unofficial content includes the Plane Shifts, One Grung Above, and many others. To find out if something by WotC is unofficial, the best bet is to look into the D&D Beyond list of sources and inside the Adventurer's League documents.
Official content published there by Wizards include the Tortle Package, Volo’s Waterdeep Enchiridion (an excerpt of Waterdeep Dragon Heist), Winter's Splendor (an official AL adventure themed around Christmas), among others.
Wizards of the Coast as Publisher
DMsGuild Adepts is a special category of unofficial content published in the Guild. Products under this label are still considered unofficial, but they are loosely endorsed by Wizards of the Coast. This kind of content will always include the DMsGuild golden logo in its cover, and some of the adventures published under this category are considered AL-legal as well.