What do I say when an opponent asks if I'm "tapped out", especially if I still have a means of casting a spell?
First, let me tell you about your obligations with regards to providing information.

There are three categories of information: free, derived and private.
Free information is information to which all players are entitled access without contamination or omissions made by their opponents. If a player is ever unable or unwilling to provide free information to an opponent that has requested it, he or she should call a judge and explain the situation. Free information includes:

[...]
The physical status (tapped/flipped/unattached/phased) and current zone of any object.
[...]

Derived information is information to which all players are entitled access, but opponents are not obliged to assist in determining and may require some skill or calculation to determine. Derived information includes:

[...]

Private information is information to which players have access only if they are able to determine it from the current visual game state or their own record of previous game actions.

[...]


Also,

At Regular REL, all derived information is instead considered free.

If he's seeking to know which of your permanents are tapped, you are obligated to answer.
If he's seeking for more than that, you are only obligated to answer when Regular Rule Enforcement Level is in effect. (This is the REL used at events such as FNM.) Determining which of your permanents are lands or which of them have mana abilities is derived information.
Whether you are obligated to answer or not, all answers must be truthful and without omission.

On to your question. Given that he didn't mention lands or mana, I see three alternatives.

You can could ask what he means.
You could overload him with information. For example, &quot;I have many untapped permanents. J, K, L and M are untapped.&quot; For all you know, he's asking if you have any potential blockers. If not, &quot;Oh, you just wanted to know about land? Dully noted.&quot;
You could misdirect while being fully truthful. For example, &quot;only non-lands aren't tapped&quot;.