I believe the term would be "gravid" rather than "pregnant."
Sometimes you can palpate right in front of a back leg and feel eggs, but unless you know what you're feeling its pretty hard to tell.
When a female tortoise is ready to lay eggs (normally happens in the spring, for hatching around September) she gets restless, won't eat and maybe digs a few test holes. But chances are good, if your tortoise is outside, she's not going to lay eggs now. She'll wait until spring.