It depends which cabinet your line Is fed from. The cabinet being the green box usualy 4 ft high typicaly on the pavement in your locality. If on Infinity then there will be a new cabinet next to it. Usually with a small openreach sticker on it stating a new type of service is avalable.
the next problem is should you spot such a cabinet you dont know for sure if its the one that feeds your property. Or if your line may be fed via a secondary cabinet which may be a "pillar" or the cabinet your looking at isnt a bt cabinet at all.
So this is only a partial guide. Where I used to live without knowing for sure I would have assumed my line ran via a nearby cabinet which I often passed, or maby a second one a little further in another direction. Not he one I was actualy routed through which was tucked away in a 3rd direction. In my parents case, its easy the only one anywhere visible in the area is less than 200m on the same road. Highly likely it was this one they were fed from. Sure enough a new cabinet appears and some work is seen being done feeding cabling underground off into the distance.
they had just over 1mb, I finaly sort out the star wiring, then 3mb on a very line now on infinity 38mb as measured via an online test. Not only can I-player work uninterupted but 2 computers on i-player hd and a far more reliable skype all at the same time.