The short answer is that there is no way of knowing without asking. There was a time when different networks had different first 5 numbers. However, since people have been able to switch their old number to a new network, this isn't as easy as it once was.
You do need to bear in mind that when you do change network and keep your number, when it comes to calling the mobile from home, you get charged the rate for the network that originally issued the number. 3 network is the most expensive to call from a land line, and say for example you get a pac code and transfer to orange, when you call the mobile number from your home phone, you will get charged for ringing a 3 network number, not an orange number. It's a long time since I have seen a call charge chart for obvious reasons, but 3 is by far the most expensive network to call from a landline.