RW, most AI failures are due to vac pump failure and can be quite insidious (if you don't notice the loss of suction/pressure) or have a light to draw your attention to the problem.
I have had several AI failures in IMC. The insidious ones were picked up because of the slight disagreement between instrument indications as the gyros wind down. A good scan will pick this up quickly.
Once you indentify that the AI is a dud - you need to get it covered quickly - but make sure you have the aircraft S&L on partial panel first. It is surprising how off-putting a wonky AI is to your scan and control.
Even in the FTDK with an electric standby AI close to the real one, it takes concentration until you get the wonky one covered.
Then there was the time that I just happened to be looking at the AI when it rolled upside down and stayed that way - not too hard to pick that one up.
Dr 