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;

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

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

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

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

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

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

ZenScript
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//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);
    }
}

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

ZenGetter	What does it do	Rückgabetyp	Usage
ZenGetter id	What does it do Returns the mod's id	Rückgabetyp string	Usage mod.id
ZenGetter name	What does it do Returns the mod's internal name	Rückgabetyp string	Usage mod.name
ZenGetter version	What does it do Returns the mod's version	Rückgabetyp string	Usage mod.version
ZenGetter description	What does it do Returns the mod description	Rückgabetyp string	Usage mod.description
ZenGetter items	What does it do Returns all items added by the mod	Rückgabetyp IItemStack[]	Usage mod.items