Depends a lot on your relationship with the asker.
If it's the CEO, you should probably just give the information and not worry about it.
If it's a direct report to you, you should probably care a lot more (since it is somewhat your job to actually coach/train people to not do this).
If it's a colleague, you have a vested interest in them not becoming a help vampire.
- "Hi JoelFan, can you give me this piece of information?" (this information is easily accessible from email conversation or online resources)
- "Sure, I think it was in the email conversation about XXX - I'm not able to look it up now but you could check there"
- "Hey John, have you looked at the email from Joe? I think he answered that - I'm not sure off the top of my head, but I would start there"
- "I think (insert resource name here) has that - I would try looking up (insert search term or process here) - let me know if you aren't able to find it there")
If you get another request on this with no effort, just forward the original communication- I wouldn't worry about being rude at this point, you've already told them it's in that information and they could find it themselves.
Simply delaying your response (via the above) causes people to be more likely to actually look stuff up on their own in the future. You doom yourself if you become an "immediate answer" location regardless of what you are doing.
I have a friend who I've helped enough I just ask him, "what have you tried?" or "what did Google tell you?" when I don't think he's done much work himself - because we're close enough this works for him and does not offend him. I often get back responses after some time of, "found it, thanks" even though I didn't actually do anything at all. Depends on your relationship though.
Most of the other answers involve enabling someone to ask more of these out of convenience.
You do NOT want to enable people to ask these types of questions more often. Simply giving an answer without trying to change the way people approach questions is not going to make them go away in the future.
You have to respond, consistently, with answers which force people to learn themselves that, "if I ask JoelFan a question, JoelFan is going to recommend I do this or that to solve it myself first" is how that person learns.