This is the correct answer.
To expand on it:
You don't need a fancy machine. Lenses, including our corneas, work both ways.
Therefore you can just shine a narrow beam of light into the eye, moving it across the eye serveral times (Skiascopy). If the beam moves across the retina in the opposite direction of the movement of the light, you know the focal point lies in front of the retina.
If it moves across in the same direction as the beam, the focal point lies behind.
Now you can just hold up test lenses and see with which lens the reflection of the beam of light stops moving, meaning the retina is in focus.
This is basically the old school method of doing it, but is much easier explained by drawings or a video.
Coffeeaddict444 t1_j205x7m wrote
Reply to comment by AirborneRodent in ELI5: When scientists say babies have blurred vision, how do they know? by DreadMCYT
This is the correct answer. To expand on it: You don't need a fancy machine. Lenses, including our corneas, work both ways. Therefore you can just shine a narrow beam of light into the eye, moving it across the eye serveral times (Skiascopy). If the beam moves across the retina in the opposite direction of the movement of the light, you know the focal point lies in front of the retina. If it moves across in the same direction as the beam, the focal point lies behind. Now you can just hold up test lenses and see with which lens the reflection of the beam of light stops moving, meaning the retina is in focus. This is basically the old school method of doing it, but is much easier explained by drawings or a video.