I can answer a few of the questions here at least.
I'm from the UK and visited my GP some months ago about starting finasteride, he didn't know the drug very well and refreshed himself on it right then and there as I sat in his office.
Anyway I asked for a DHT test at that visit and my GP pretty much laughed at that request, saying that the NHS don't really perform DHT blood tests unless there's highly warranted reason for it. I'm not sure if 'I'm trying to find out if finasteride is really working' would constitute as being a 'highly warranted' reason in a GP's book!
DHT blood tests are expensive which personally I believe is due to the fact that it's not a commonly required test for discovering the majority of health issues, hence not many labs bother to aquire the necessary equipment to do the test, hence the ones that do, charge a pretty penny.
I had my DHT tested (baseline i.e. before starting finasteride) privately, and I paid £99 as I had other blood tests done at the same time so I got a multi-buy discount (the DHT test being discounted down from the full price of £119 if I recall correctly). It was done with a well known UK testing lab who have the website: medichecks.com
My DHT was in normal range but in the 'low end' of said range. Being at the low end in terms of DHT whilst still having hair loss, cements the fact that the sensitivity of your androgen receptiors counts for a lot in the world of balding.
As regards testing whether or not finasteride is working with only a 1 week break-off from it, hhmmm that might be tricky as I'm not sure how many days/weeks it takes for DHT levels to rise back up to their normal pre-finasteride state - any takers?
It would be quite a bit easier to test whether or not finasteride is working as regards DHT inhibition if you had some baselines result to go off of, but to be honest I think it would be fairly pointless, I mean if the tablets are legitimate and/or not damaged in any such way, then I would be pretty certain that they will be lowering your serum DHT.
A PSA test is recommended to all men prior to starting finasteride, well at least that's what it says on the leaflet within the pack of 1mg finasteride tablets that I have. However It's my assumption that this refers mainly to men over the age of 40 as that's when PSA testing starts to become more of an annual necessity.
I had my PSA done privately too but I would of been able to get it done on the NHS for free if I wanted to, but for the sake of making appointments to request it and then getting it done, I thought it easier to just get it done from the same blood samples as taken for my DHT test, as the PSA test is relatively cheap anyway. PSA score can be used as another indicator that the finasteride tablets in which you are taking are actually doing what they should be doing, but again without baseline scores to go off of, it might not prove all that much, and once again I can't imagine why any genuine licensed finasteride tablets would not be working in terms of their effect on both your PSA and DHT levels.
As regards if it's working for your hair loss, well it seems that for most men it does work for hair loss, with only 1 to 2 men out of 10, not seeing any benefit.
But (and here's the clinch), once you start finasteride, unless you get obvious 'above baseline' regrowth, you can never truly say if it has or has not benefited your hair loss, as you will never know how 'bad' or 'not so bad' your hair loss would have been without it. Of course balding can halt, slow down or speed up and any given time, albeit it's the not knowing which of those variables it will take, that has us taking finasteride for the hope of some stability.
For myself I got a wide arrray of blood tests for the following 3 reasons:
1) I wanted a good overall picture of my serum health prior to starting what could be a life-long oral hair loss treatment.
2) If I get good results from finasteride without any side effects, then I'd like to re-test my blood again to make sure that nothing other than my DHT and PSA are showing any 'out of normal range' scores.
3) If I do get sides and have to stop finasteride, then I'd like to know that my DHT goes back to relatively near where it was pre-finasteride and doesn't either remain either very low or suddenly sky rocket and remain there.
Then again knowing too much can be a curse in itself and so I end this lengthy ramble by saying that; for the majority - ignorance really is bliss.