I also find what Skywalker has said to be a good way to approach the situation.
However sometimes no-one in the family knows even an approximate period of the day (morning/afternoon/night) the person was born. So I use those transits, progressions etc he is talking about via a software program from Esoteric Technologies called Jigsaw. If a person has reached their first Saturn Return or beyond then they will have had time to accumulate many significant life events. If they compile a list of these events they can be put into Jigsaw and it uses these dates and times to find the probable birth time based on transits, progresions and arcs aligning with the person's main life event dates/times. Then using that birthtime you can look at the chart and see how well it matches the actual person. Jigsaw also has tools to help finetune the original time(s) it came up with. But it does depend on a good list of life events the better the list the better the results Jigsaw can provide.
So I also get people to see a kinesiologist who muscle tests them for their birthtime. I "tested" her first with my time (without telling her what it was), and she was very accurate. And I feel she usually comes up with times for people that are useful. If the birth time she gives me for a person is very close to the time Jigsaw gives me for the person (which is often), then I use that time with a lot of confidence.
However out of interest I've tried different kinesiologists with this, and found some are much more suitable than others, in that they seem to come up with better times, times that either make a lot of sense when I look at the person and/or correspond to a Jigsaw time I have gotten for the person. So not all are equal in this regard, I'm fortunate to have found a kinesiologist who seems to be able to use muscle testing very skillfully for this purpose.
Then, as Skywalker has said, if you know the person well, you can take planets on the cusps of signs etc (moon is a good example here as it moves quite fast) and play with the either/or possibilities of what sign it is in by observation, correlation and talking with the person. Intuition is very useful here. Then finally any new birth time I find for a person can be further confirmed by watching for current transits to sensitive points in the chart - eg ASC, Moon etc.
As you mentioned the Pluto questions can be used too. Sometimes these are useful, sometimes not - depends on how much a person is in tune with their Soul energies, but also how their chart is constructed ... can often have complicated aspects etc invloving Pluto that mask it's true position for either the person themselves, or me asking them questions. But it's an important tool in the toolbox.
If I cannot decide what ASC a person has, I get them to read Martin Schulman's detailed book on Ascendants and say what they feel is the best fit for them, and this gives me additional clues.
The whole process can be a lot of work (not always though) but there seems no other way. And once a person has a good "working" birthtime it makes the houses available which is invaluable, especially in EA.