Recently a friend of mine asked me,on seeing a LAN,how to find its type?, the only answer i could give her was , on seeing the cabling used, we can say whether it is 10 Base T, or fast ethernet etc.
Perhaps one question to ask is what do you mean by "on seeing a LAN"? You mean, the cable? The hosts communicating together? The switches?
If you mean that you see cable or wall sockets only, no attached hosts, one easy way might be find the wiring cabinet and physcially look at the switch(es) used to fan out to the various rooms. Look at brand name and model number, and proceed from there.
If there are hosts connected, then you can always get at least an idea by looking at the "properties" of the NIC in a few of the hosts. The typical choices these days are not all that varied.
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