Bourne Shell: Variable defined?

[Rouben Rostamian]

In C Shell the variable $?foo returns true or false (0 of 1, really)
depending on whether or not the variable "foo" has been already defined.
What is the equivalent construction in Bourne Shell?  In other words,
how can I tell, in Bourne Shell, whether the variable "foo" exits.

Sorry if the question is too elementary;  I am new to unix.


-- 
Rouben Rostamian
Department of Mathematics                      e-mail:
University of Maryland Baltimore Counnty       Rostamian at umbc2.bitnet
Baltimore, MD 21228                            rostamia at umbc3.umbc.edu