This might be hard to know, but good to know if you can. If you are a postdoc, this is even more important because the chances of you landing on a faculty position depend heavily on the recommendation you get from the advisor.
A good boss will be critical of your writing style, and regularly point at things that you can improve, but will speak proudly about you with others. He will introduce you to fiends and fellow researchers, and help you to create a good network of connections. I have had two of these during my research career, both of them before I started my phd.
A bad one will butter you up once in a while, but interesting things can happen when he talks about you with others. Here is one of my experiences.
I was a "research assistant professor" (an assistant professor hired for a short term using a research grant). My boss, the principal investigator of the research project, was a middle aged professor. Once we attended a social session at Experimental Biology 2011, to present a paper. I was the first author, and the boss wanted to attend because of the opportunity to meet researchers and experts.
On the day of the conference, we reached the conference venue together. We reached the venue of the session, about 30 minutes earlier. The next step was to introduce myself to the session chair so that she does not have to worry, but she was not in and there was plenty of time for that. So, I went to the lounge to get some fuel (I mean, coffee :-p).
When I came back, the session chair was in and talking to my boss. So, I approached them and stood there waiting for my turn. My boss, who did not see me coming in, kept taking; "the paper will be presented by one of our post docs". "You mean him?" asked the session chair, pointing to me (i guess like all good session chairs, she did her homework and googled the home pages and photos of the authors).
I put on a poker face, and the boss turned red. "Actually, he is an assistant professor." Then the session chair started taking to me, before we all sat down and started the session.
In fact, the prof used to be a generally nice person. However, he created a startup attached to this project, and seemed to think that researchers and students in the project should help his startup rather than focussing on research. I guess this made him a bit grumpy. I could also be that he wanted to use the contacts for the benefit of the startup, and avoid others from contacting me.
I wasn't happy to know that I am regarded as a postdoc, when i was working hard as an assistant professor. I am sure it was no mistake, given the short time it took to correct the mistake and his facial expression. For me, I am not the type who would take this kind of thing face to face. Instead, I decided to change my job soon.