It should be on the individual website for any hotel in the Marriott portfolio.
Yes, it SHOULD be, but it usually isn't.
The official Marriott.com webpages for some hotels proudly show photos of a Concierge Lounge, Signature Lounge, Executive Lounge, M Club Lounge, Delta Pantry or whatever -- while other hotels (sometimes including hotels with such lounges) ignore lounges. Even if a hotel's webpage indicates it has a lounge, there are seldom details about whether it's open on weekends and what services it provides.
Guaranteed Lounge Access is a benefit for Platinum Elite members (and above) at hotels (but not resorts) branded as Marriott, Renaissance, JW Marriott, Autograph Collection, and Delta Hotels. If there's no lounge or the lounge is closed, then the member can choose breakfast in the restaurant for member +1 or 750 Marriott Bonvoy points. In the U.S, and Canada, it only has to be continental breakfast, but most properties provide a full breakfast. There are a few exceptions and limitations in the Marriott Bonvoy Terms & Conditions.
Guaranteed Lounge Access is also a benefit for Platinum Elite members (and above) at hotels and resorts branded as Sheraton, Le Méridien, and Westin -- but only if there's a lounge. If there's no lounge or it's closed, then there's no alternative.
Guaranteed Lounge Access is also offered at some Courtyard properties in Asia.
Marriott Bonvoy brands not listed in this reply do not provide Guaranteed Lounge Access.
Some brands -- notably those that came from the Starwood merger and resorts of five Marriott full-service brands -- offer daily breakfast in the restaurant for member + 1 as a Platinum Elite (and above) Welcome Gift choice.