Is this only way can I check the estimated speed of my ADSL line?
Well, more-or-less. You could figure out the route that the line takes back to the exchange, then measure that distance (walk or cycle with GPS or drive and use the odometer). Then look up the likely speed using this graph
but a postcode based checker will probably be more convienient.
Another approach, which you seem to have taken, is to talk to the neighbours and see what they get.
Remember that the checkers tend to be a little pessimistic, it's not uncommon to see actual connection speeds higher than the estimate.
At the end of the day though all lines are different and you won't really know until you get an ADSL service on that line, however it looks as though you will get roughly 6-8Mbps. Is you friend on ADSL2+ or ADSL?
You seem to already have had the answer you need
before you posted. What is it that you want