This is a complicated codebase and it can be difficult for developers to how to check something or get a reference to what they need. This is a cheat sheet of common functions to help you quickly find what you need so you don't waste a lot of time looking or reimplementing something that already exists. This is not intended to be a complete list of functions, but just the most commonly used ones. Please expand it or modify it as you see fit.

To write to the debug log:

Logger.Log(string msg, Enum Category)

To set the log level for different categories, go to unitystation/UnityProject/Assets/Scripts/Debug/Logger.cs, and modify

private static readonly Dictionary<Category, Level> LogOverrides = new Dictionary<Category, Level>{
		[Category.Unknown]  = Level.Info,
		[Category.Movement] = Level.Error,
		[Category.Health] = Level.Trace, 

to your liking.

Get player info by gameObject (serverside only)

ConnectedPlayer player = PlayerList.Instance.Get(gameObject);
if (player != ConnectedPlayer.Invalid) {
    //Do your thing. If gameObject is not found in PlayerList, ConnectedPlayer.Invalid is returned
}

Check if player has spawned

bool playerSpawned = (PlayerManager.LocalPlayer != null);

Check if local player is dead

PlayerManager.LocalPlayerScript.playerHealth.IsDead

Check if local player is in critical health

PlayerManager.LocalPlayerScript.playerHealth.IsCrit

functions related to items the player has equipped

PlayerManager.Equipment

Functions related to player taking actions with the server

PlayerManager.LocalPlayerScript.NetworkActions

Check if this is the server, but only inside a NetworkBehaviour

if(isServer)

Check if this is the server

CustomNetworkManager.Instance._isServer == false

Get the room number of a tile (only enclosed spaces will be defined as a room. RoomNumber will = -1 if it is not a room)

MatrixManager.GetMetaDataAt(Vector3Int.RoundToInt(world_position)).RoomNumber

The Assets\Scripts\Util folder also contains a bunch of useful scripts to help deal with common problems. Have a look through there and you might find something useful!