Echo the level of subscription a user is on

Hi,

I have read posts that give these codes:

current_user_is_member()
current_user_has_subscription()
current_user_on_level($level_id)
current_user_on_subscription($sub_id)

I am trying to Echo the ID for the level of subscription a user has but echoing the above do not seem to be working. What is the exact code to echo the subscription ID? I have tried

echo current_user_on_subscription($sub_id);
and
echo $sub_id;

Please help, this is driving me nuts! haha