We recently adopted a cat from an acquaintance who is about to move
 out of state and cannot take her cats with her.  We live in an urban
 setting --San Francisco-- and have a house with a small, fenced in
 back yard.  When we've had a cat in the past, we would often leave
 the back door of the house open so that cat could come and go at his
 or her whim.

The new cat has been in the house for two days and is only now
 occasionally coming out of hiding.  What I'm wondering about, is
 what kinds of behavioral signs I should be looking for to indicate
 that it would be safe to let the new kitty have outdoor access.
 She's the only cat in the household.

Thanks.