Indications that you could be include:

• Being off your Food
• Being moody
• Exhibiting "nesting" behaviour
• Restlessness
• Bleating

Attribution: 

The Land (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

This has been a public service announcement courtesy of Edinburgh Zoo. More details can be found here.

Andrew