Whenever I play, I think I sound about the same as the guy next to me but today the french hornist gave me a funny look after we were done with a piece. If the sound you hear, is the NOT the sound others hear, whats a good way to figure out what others hear?

Good question!
The problem is, more often than not, we hear what we want to hear, not what's actually coming out of our bell. This happens for two reasons.
1) You're on the other side of the bell, plus you get your own head noises, etc interfering. But this difference is MINIMAL.
2) The real issue is that we don't listen critically and hear what we want. I'll give you an example.
A student of mine was playing one of his excerpts and the metronome was going right along and he was way off time. So, I asked him to play it again, and this time, to play in time, with the metronome, not in spite of it. He played again, and was still way off. He didn't understand what I was talking about! SO, it was time for tough love, and I recorded him and made him listen to it. His eyes got really big as he heard it.
It's very common to think we're playing in tune, and in time when we're not. We think we're sustaining a note full value when we're actually cutting it off early. The articulations you may feel you are exaggerating may be very subtle to the audience.
The key is to record yourself at regular intervals. Don't just record your solo, or your excerpts. Record your daily routine, your lip slurs, your long tones. Then listen later in the day when you can be objective. Are you lip slurs played evenly? Do all notes speak well, or do some get swallowed up? Are your long tones even, with a great sound? Or do they swell and wobble? Are your notes starting right on the beat with the metronome? Or are the just a tad late every time? Are the attacks clean? Or are they flubbed?
The more you record yourself and start having some personal accountability for these things, you'll listen for them as you play and the more realistic your impression of your own playing will be. Awareness is the first step.
Just out of curiosity, did you ever just ask the horn player why he/she gave you the dirty look? Maybe they just heard one little weird note? Who knows.

Excellent Steve !!
Another thing you might want to consider is that even when listening to the recording of yourself there will be some 'sympathetic vibrations' in your head (so technically you can never really hear what you sound like) - but like Steve said, it is minimal. Anyway, find someone you trust to listen to your recording every now and then and get their opinion. As far as tone quality goes you might want to play 3 or 4 different trombone recordings for a trusted freind and ask which one you sound most like. This will give you an idea what your tone sounds like and you can make any adjustments if needed. It's a bit of an extreme, but if you have a sound color you are going for . . . . .

This question is alot like when i see a certain colour does the other person see the same thing? i would guess in terms of sound we do probably hear differently depending on what our ears can pick up due to use and such, the environment that your in and the way sound moves around, and also what part of the sound your brain is concentrating on..i'm no scientist but thats what common sense tells me

If you are tlaking about colors directly, you are correct we do not see colors the same way - this explains why some people who get Lazic surgery love it and some hate it because the 'cones' of the eyes areaffected in such a way that it affects the colors they see.

Whether it is color or sound both are interpretations of vibrations. (yes colors have sound). If you see colors when you hear music then you have Synesthesia. I study in school with a man who had Synesthesia - it was wonderous hearing him explain what he was seeing. Different people with Synesthesia will see different colors - so sound is absolutely the same way.

Ok, Ok... but read the original post...
The guy says basically " I think I sound fine and others apparently do not" So what's really getting asked? Do you really think this is a case of synesthesia, or acoustics in the room?
I don't.
This is probably someone who isn't practicing enough on his or her own. When they play in a section they hear the sum of the parts, not their individual contribution. They hear what they think they are playing. As far as what I read from the original question, this is a case of someone needing to develop better critical listening skills, someone who needs the harsh medicine of a self recording session.