LoadedMods

You can use the global keyword loadedMods to access all currently loaded mods.
If needed you can import the class using import crafttweaker.mods.ILoadedMods;

Check if a mod is loaded

Use the in function to check if a mod is loaded;
You can also use the contains method:

//if MinecraftCoderPack is loaded
if(loadedMods in "mcp"){
  print("success!");
}

//if MinecraftCoderPack is loaded
if(loadedMods.contains("mcp")){
  print("success!!!");
}

Retrieve a specific mod

You can retrieve a specific mod as IMod as long as you have it’s ID

//retrieves the minecraftCoderPack mod
val mod = loadedMods["mcp"];

Iterate through the modList

You can iterate through the list of loaded mods like this:

//prints all registred item definitions, not recommended in larger packs!
//the mod variable will be an IMod type
for mod in loadedMods {
  print(mod.name ~ ":");
  for item in mod.items {
    print("\t\t" ~ item.displayName);
  }
}

IMod

The IMod Interface provides you with some general information on a specific mod.
If needed, it can be imported using import crafttweaker.mods.IMod;