Without being able to manipulate gravity.
We are manipulating gravity all the time, except on earth, labs and constructions do not allow timing gravitational effects, which is why newtonian gravitational theory which has instantaneous effects is so successful.
How do we know that gravity is restricted to the speed of light? or gravitational effects from an exploding star would be felt (if we were near) before we see it?
We know with the mathematics of General Relativity. In physics when we have a mathematical model that fits existing data and predicts accurately new situations we consider it as good a "measurement" as reading off a thermometer is a measurement. The satellites which give the GPS locations use General Relativity to work properly, so even at a planetary level the model is validated . This has inherent the limit of the velocity of c for transmitting gravitational effects.