I'm working on a story involving first contact with an alien species that bases their space travel on directly manipulating gravity fields. My question is not about how that would work, but rather how would you be able to DETECT it working?
More specifically: My intrepid human explorers are operating in at a technology level sufficiently advanced to allow interstellar travel, but not advanced enough to involve Faster-Than-Light technology of any kind. They are newly arrived in a previously unexplored system, and during the encounter with previously mentioned aliens, the aliens start moving the human's ship.
So: If you're in interplanetary space (e.g. not close to a planet), and something creates an artificial gravity well which alters the orbital trajectory of your vehicle, how would you know what had happened?
Obviously if you're paying close attention to your relative position with the planets and the star itself you'd notice that SOMETHING had altered your vector, but what other instrumentation would notice?
The ideal answer would involve something that generates a "Well of COURSE any reasonably well-equipped scientific spacecraft would have one of those." reaction from the reader, rather than a "Wow, they're lucky they had one of those that they probably never thought they'd need or use."
EDIT: You should be imagining the Endurance from the movie Interstellar, except mine isn't specifically exploring a black hole, so my ship would be even LESS likely to have specialized instrumentation to detect gravitational anomalies.