As others have mentioned, ESRB is often a good place to start, especially if you just want a high-level overview with a short, bullet-point list of objectionable content.
If you want something more in-depth, you might try Common Sense Media's reviews. These are targeted at parents, but may also be useful for anyone who may be sensitive to particular kinds of content. See for example their page on Hitman 3, which provides a much more detailed description of exactly what sort of content you can expect to see in the game. Note, however, that their reviews do tend to contain spoilers, as it's impossible to discuss a game at this level of detail otherwise. Here's a short excerpt of their "parents need to know" section for that game:
Players will typically kill far fewer characters in this game than in most M-rated action games, but the assassinations can be quite graphic and intense. Depending on the manner of death, killed characters may bleed, scream, or flail. Some examples include: garroting, getting crushed by a chandelier, being drown in a toilet, and falling from a skyscraper balcony.
The actual review is longer and provides additional context about these scenes, including the morally gray protagonist. It also provides a set of numerical ratings (out of five) for various different kinds of content, such as violence, sex, language, drug use, etc. It also contains a more traditional review (i.e. answering "is the game good?" in addition to "can my kids play it?"), but this segment is rather shorter than what you might expect to find in a typical review.
Finally, readers need to know that Common Sense Media is an advocacy group, which has at times taken positions that I would consider questionable if not problematic. Nevertheless, in my experience, their reviews tend to be relatively well-written and fair even though I'm not a fan of their politics. All the same, I would encourage readers to carefully consider the way that their reviews are framed and characterized rather than blindly relying on their numerical scores. It may also be a good idea to read Common Sense's reviews for games which you have already played, and see whether they are consistent with your personal experience with those games.
