Not all lenses have a depth of field indicator! It seems like it is a thing of the past, so you need a good old lens to find a DOF indicator! And that is not necessarily a good think, as it can be very useful!
On older manual focus lenses there is a distance scale where a – typically – white marker shows what distance your are focusing at. The scale is often very detailed for close distances, and as you increase the distance the jumps get bigger and bigger: 5 meters, 7 meters, 10 meters, 20 and infinity. So it is like a hockey stick – when you get to the far end, things happen very fast!
In addition to the distance scale, you also select the aperture on an older lens by turning an aperture ring. Each of the f-stop values are color coded – if you see the image below, the f/16 value has a pink (?) color coding.
Now here’s the trick: On both sides of the white marker that shows the focus distance, there is a pair of colored markers that shows the near and the far end of the DOF. In the example above to the left at f/16, you can see the two pink arrows showing the DOF markers and in the example to the right it is green arrows doing the same for f/8. The image above is from the video that I have linked to below, in case you are interested in more details about manual focus.
As you can see on the colored pairs, the blue markes give a much wider DOF than the green markers, which is not a surprise as the blue corresponds color wise to f/22 whereas the green corresponds to f/8.
The DOF scale can be super useful to determine the circle on confusion, i.e. the range in which the image appears to be sharp. This for example enables you as a landscape photographer to put the focal plane so that the far end of the DOF just touches infinity on the distance scale, and hence you make the best use of the DOF at a given aperture. It can also be a group portrait where you want to select an aperture so that everyone in the image will appear to be sharp, and not just the front row!
Related reading
What is manual focus in photography?
Using a grey card and histogram to exposure correctly in Manual mode