I considered how I myself (and how early) develop a belief a client may be mentally unstable/have a condition of mental illness.* I do this through verbal interactions with a client and sometimes suspect he's mentally ill from our intake (first) appointment. Typically this is because I'm unable to have a logical conversation with him or he shares delusional beliefs with me. His mannerisms and affect are involved too but what he chooses to share and how he shares it are important factors I rely on.
What about other conditions though? It's possible to detect some on a short encounter, like whether a person is drunk or developmentally disabled. This is especially the case if one is familiar with people who have similar conditions. On the other hand, it's not difficult to make mistakes when assessing a person's behavior and facial expressions in a few or even several seconds. I haven't seen studies that explore length of time in regards to correctly assessing mental illness but it doesn't seem like it'd be possible to accurately assess a person's sobriety or whether she's developmentally disabled without observing her for a longer period of time (1-2 minutes) or having a short conversation with her.
In Ian's case, I'd also predict that the fact he works almost exclusively with people with schizophrenia means that he's primed to find this illness, whether in a person who has mental illness or not. I may understand why he'd be more sensitive in recognizing this condition but not sure if he'd be able to do it with higher accuracy than a non provider. I'd expect though that he'd have a deeper understanding of what the condition means.
Because I couldn't help myself: Of course this could start a whole conversation of DSM IV and how mental illnesses are assessed (clinically) in the first place but that's for a whole other discussion...
* A reminder that I don't have a clinical background and so don't have credentials to assess mental illness.