I think that similar behaviors would be key to look out for - lack of interest, lack of expressions of joy or pleasure from something that used to elicit those reactions... The level of impairment in other functioning also plays a role... are they more agitated than previously? is there a generally high level of agitation?
You did not say if there are any other impairments that would impact the diagnosis... is the person severely physically disabled as well?
If you are worried about depression, speak with their doctor or other provider. They would be able to help make a diagnosis, refer the person out to another specialist, or put your fears at ease...