The absolute key to know if ANY treatment is working would be to find out that things get better after such treatment started, in this particular case, you want hair-loss to decelerate.
In order to register a change, you need to first register the previous state of affairs, so you'd have to calculate hair-loss rate both before and after the treatment was started.
First of all you wanna be sure it is actually a problem or if it isn't at all.
If the rate of hair loss is not accelerating in a way that indicates some pathological problem, then there wouldn't be anything to worry about because humans normally lose some hair and grow some new.
Otherwise, if the hair is falling in an accelerating manner and the amount of new hair does not correlate, then a diagnose of the cause and a treatment may be necessary.
The cause is beyond the scope of this question so I'll stay on topic by elaborating on measurement and calculations.
This calculations shouldn't be done by means of human perception (e.g. photographs or simple observation) you want a method that's less fallible, there are other suggested "practical" methods like wearing a cap all day and then count the amount of hair that stays on the cap when you take the cap off and when you take a bath, this kind of methods are messy and there are multiple chances where mistakes can be made, so I'd say error-prone methods like this are useless. Not to mention, wearing a cap at all times is not practical and may even make things worse.
I found some very interesting articles licensed under Creative Commons in the National Library of Medicine.
The first one describe a variety of methods, ranging from non-invasive to semi-invasive to invasive.
e.g., Questionnaire, daily hair counts, standardized wash test, 60-s
hair count, global photographs, dermoscopy, hair weight, contrasting
felt examination, hair feathering test, phototrichogram and
e.g., Trichogram and unit area trichogram (UAT).
e.g., Scalp biopsy.
A scalp biopsy seems way to invasive and not suitable to be doing it frequently in order to calculate a rate of acceleration/deceleration, so I investigated further on the Thrichogram option; it turns that method has been used and approached from different angles since the 80s:
This personal technical adaptation gives an easily obtainable definition of five quantitative parameters analysing the hair on a given area of the scalp: density of implantation, telogen percentage, growth rate, mean anagen diameter and percentage of fine hair less than 40 mu in diameter. In the normal male adult our measurements gave the following results on the vertex: density 204 +/- 10 hair per square centimeter, telogen percentage 17.8 +/- 2.8 p. 100, growth rate 0.35 +/- 0.03 mm p. day, mean anagen diameter 76 +/- 5 mu, percentage of fine hair 9.2 +/- 1.8
And there seems to have been some progress recently in the development of instruments to aid in such methods:
To properly assess the progression and treatment response of alopecia, one must measure the changes in hair mass, which is influenced by both the density and diameter of hair. Unfortunately, a convenient device for hair mass evaluation had not been available to dermatologists until the recent introduction of the cross-section trichometer, which directly measures the cross-sectional area of an isolated bundle of hair.
The article mentions one particular trademarked device but there must be others. So there you have it, I'll venture to state with some confidence that the best approach is to use some device like that one and measure your scalp at least one time a day. And for security sake and professionalism sake, I'd recommend some expert assistance from a dermatologist. But if you have a Scientific Mind and a Hacker Spirit, do it yourself, why not.
(Just the measurement part, don't diagnose yourself.)
So in this particular case, if vitamins don't decelerate your hair loss rate, then to hell with vitamins, you may need something else, so go to a doctor and get properly diagnosed.