Often when we are asked Shidduch information we are stuck between the Halachos of Lashon Hara. Suppose there is an illness you are aware of, however it really is not something that would affect a marriage what do you say, how do you say, when do you say?
A point to remember when there is reason to discourage the shidduch is that Lashon Hara for a to'eles is permitted only when there is no other way to achieve the same constructive outcome.
My Rosh Yeshivah told us that as a teacher of many eligible bochurim he often received inquiries about the suitability of a particular student. If there was indeed some defect in the potential shidduch he would look for some other innocuous reason to discourage it. The age gap is too large, they have very different backgrounds, etc. rather than elaborating or even hinting that a shortcoming existed. Thus he would prevent the shidduch without resorting to Lashon Hara.
There are a few different cases here:
A. The guy/girl is hiding a severe condition, one that any normal person would need to know about (e.g. untreated severe bipolar disorder). You'd be obligated to proactively tell the other side about this (assuming you're not bound by whatever professional privacy laws), assuming you know what you're talking about, and the result will be that they'll discuss it and possibly call off the shidduch (which is proportionate given the problem). If they'll shoot the guy/girl (disproportionate response!) on one hand, or if on the other hand they'll ignore you, go forward with the shidduch, and let the doubt and resentment fester, then telling them won't accomplish anything productive. Certainly anything so bad that would require a divorce, or render the wedding void due to gross misunderstanding (mekach ta'ut) is certainly bad enough to call off a shidduch, or even an engagement.
B. There's a condition that most people don't care about, but these people do. Shprintza has just committed to a ten-year job program in Chicago -- normally, not something you (outside party) need to be proactive about. But if Chaim calls you up and says "I absolutely must move to eretz yisrael the day after the wedding no matter what, by the way, anything I should know about Shprintza?", then it's a situation where Chaim needs to know, so you tell him.
In both these cases, it's incredibly important to get your facts exactly right (or if it's substantiated hearsay, present it as such) and maintain a neutral tone. I think putting your statement in writing is a good way to do this.
C. You're asked about something ridiculous, that really shouldn't matter for anyone normal. If Chaim doesn't find out about it, he'll have a perfectly fine marriage, but no he's being foolish and asking you anyhow. (Or worse, Chaim is immature and making a non-issue into a dealbreaker. Maybe he's just not ready yet.) E.g. "is she a size zero?" You can get put into impossible situations here. If you can say "how should I know?" and he'll believe you, that's probably your best bet. Otherwise ... I guess you say "Chaim that's none of your business"; if he calls off the shidduch because of that, well, his loss and Shprintza's gain.
D. You're asked something vague, "is he smart?" Depending on the circumstances, this could be A, B, or C. Tread cautiously -- and ask them to be more specific.