How do you know what mobile network someone is on

The way you can change networks and keep numbers how can you tell for example that someone with 085 is definately meteor and not 02, is there a way?

When you make a call there will be a beep before the ringing tone starts if are dialing another network, at least that is the case with vodafone if you dial an o2 or meteor network phone.

Meteor have [broken link removed] for checking if a number is on their network. Maybe other network operators have something similar?

handy link that, but it's a pity there isn't an online version as I too have a load of numbers that I don't know what network they're on and would like to know as it may influence my soon to be new provider.

Also, I wonder if those texts are charged for ?