I think you can assess pretty quickly the level of the person you're talking to in your native language. Within five minutes you can work out if you can speak as you would with another native speaker and there be no misunderstanding.

Generally I do make an effort to speak more clearly and decrease the amount of idioms and slang I use i.e. switch to a more standard form of the language when speaking with non-native speakers, to try and make the communication as easy as possible.

My personal policy is to treat everyone like a native speaker of English unless there is reasonable evidence that they won't understand.  So I won't limit my vocabulary or simplify my grammar when talking to my physics professors who are non-native speakers of English, since they're completely fluent in English and have no trouble understanding me.  If I did speak more simply to them, it would be extremely rude.  But I will use simpler vocabulary and grammar and shorter sentences when I'm talking to someone here on italki who has been making lots of grammar and spelling mistakes.  I do the same thing with native-speaking children.  I talk to them like adults (except I don't swear in front of kids), unless they show that they can't understand.

Sure,when I was teaching classes in China I would usually speak slower than usual and use easier words.I think it helped them to understand me better.

One embarrassing experience I had in chatting with an American through Skype was that I just spoke in Taiwanese, which I was not very good at, when he did not understant what I said in Chinese, which was very interesting.

And, another experience was that I spoke Cantonese to those who spoke Taiwanese to me when I was a child. 

I choose the pace and vocabulary according to the person I am talking to. If I notice the person struggles a little to understand me, I will speak slowly and rephrase my words, but if I feel the person is ready for a challenge I will talk to him/her in normal speed and as naturally as possible.