Asperger Syndrome is an example of a mild autism-spectrum disorder. Individuals with this condition have difficulty with social and communicative skills, particularly those skills requiring an understanding and awareness of others' emotional states, and those requiring emotional expressivity in speech and non-verbal gestures. Usually individuals with an autistic-spectrum condition have a diminished interest in relationships with other people, and therefore prefer solitary activities.
The possibility of Asperger Syndrome should be considered, in my opinion, when the history is of social difficulties & social withdrawal. Often times these problems could be the result of social anxiety, depression, etc. but I think we realize these days that mild autistic symptoms are more common in the population. This may indeed be due to an increase in the rate of autistic symptoms over time, but it could conceivably be due as well to being more aware of this syndrome, and therefore more able to recognize it.
I have found a site with some good tests for autism-spectrum syndromes, from the Cambridge Autism Research Centre. Here is a link to the tests: http://www.autismresearchcentre.com/arc_tests
The particuar tests I find most useful are the Autism Spectrum Quotient, which is a symptom checklist. Average scores on this checklist from a healthy population are about 15 for females and 18 for males, slightly higher for individuals in an analytical scientific profession such as mathematics (typically in the low 20's), but above 30 (typically 35 or higher) for individuals with an autistic spectrum disorder.
I have a few criticisms about some of the autism quotient questions, which I think cause the questionnaire to spuriously inflate the scores of people who are not autistic at all, but rather either socially anxious (e.g. the question "I find social situations easy") , socially more introverted but still very attuned to emotions and other people (e.g. the question "I would rather go to a library than a party"), or having particular intellectual interests (e.g. "I am fascinated by dates" or "I am fascinated by numbers"). Thus, shy people, historians, and mathematicians may have scores on the autism spectrum quotient which suggest that they are more "autistic" than they really are. Here, I would define an autism-spectrum symptom to be much more specifically addressed by other questions having to do with reduced awareness of other people's emotional states, clear reduced interest in social engagement or communication, and impairment in understanding social norms in verbal and non-verbal interaction. Yet, I think the questionnaire is sensitive, with a large difference in scores between Asperger patients and control patients without Asperger Syndrome.
Also on this site there is an interesting set of tests having to do with accurately identifying emotions in pictures, sound clips, or film. One particularly useful example is the "eyes test," in which you have to identify the emotion represented by a picture of someone's eyes. Here again the mean in a non-Asperger population is about 26-28 correct, but an average of 22 in Asperger's patients. If you try this test yourself, I suggest that you review the instructions first, and also make sure you are familiar with all the vocabulary words used in the test (these are all available in the downloads). The test could be biased against individuals who are just a bit less familiar with the vocabulary words used.
It is on my list of things to write about to discuss the issue of autism-spectrum disorders further, since it is a theme that comes up not infrequently, especially in a university population. Part of the discussion could include discussion about things that might help (such as social skills training, etc.) but also of understanding the issue, in its mild forms at least, as a character variant with a variety of positive aspects for the individual and for society, rather than as an overt "pathology." In any case, I think understanding and discussion will help.