I guess it's different if what you want to know is simply that they *are* thinking about you, but for all other purposes, it seems more practical and easier to get an accurate reading if you reword the question so that it's not in the present tense.
Like, "What do they think about me?" or "How did they feel about that conversation/date/whatever we just had?"
That way, even if they are not currently thinking about you, you will get an answer related to their thoughts regarding you. I would think that, if you leave it in present tense, and the cards try to tell you he's not currently thinking about you, you could easily get confused with trying to interpret a simple "he's not" into some actual thoughts going through his head.
Also, in some circumstances, a deck might hand you a reading about his present thoughts *in general*, as a response to that question, even if they don't apply to you, which could also be dangerous. Imagine if you were asking about some guy, and you got something like the 5 of cups and the 9 of swords. You could be sitting there mulling over all the terrible things that could signify for the two of you, meanwhile, he just found out that his mom has cancer, which would have NOTHING to do with you, but since you asked what he was currently thinking and that was predominant in his thoughts, that's what the cards told you.
In terms of what actual cards might indicate indifference...I dunno, I could see the 7 of Pentacles as an indicator of this. Instead of saying he is indifferent though, I see this card addressing the fact that while the querent is very excited about this person and wants to know that he is thinking about her as much as she is about him, such is simply not the case right now. The card seems to say: He is not, as you hoped, thinking about you, but keep working at it. Don't give up on him yet.