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

Link to check-if-a-mod-is-loaded

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!!!");
}

Retrieve a specific mod

Link to retrieve-a-specific-mod

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"];

Iterate through the modList

Link to iterate-through-the-modlist

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;

ZengetterWhat does it doReturn TypeUsage
Zengetter
id
What does it do
Returns the mod's id
Return Type
string
Usage
mod.id
Zengetter
name
What does it do
Returns the mod's internal name
Return Type
string
Usage
mod.name
Zengetter
version
What does it do
Returns the mod's version
Return Type
string
Usage
mod.version
Zengetter
description
What does it do
Returns the mod description
Return Type
string
Usage
mod.description
Zengetter
items
What does it do
Returns all items added by the mod
Return Type
IItemStack[]
Usage
mod.items