Dropping an object that won't get accelerated is one way to measure acceleration, but not the only way. If you are the actor causing the acceleration on yourself, as I originally intended, then presumably you already know / measure-by-doing how much you have accelerated, and you can use dead reckoning to watch the earlier reference frame fly by.

If the force is external and doesn't apply pressure then you have to drop an object.

If the force affects the entire universe the same way then that's aether and can't be measured / doesn't exist. But that was not the kind of force I was using to disprove "you can't move".