I have just moved into an Annex (renting) which has been lived in before and where the previous tenants have had broadband with BT.
I called up another company to get broadband with them, but they said that they couldn't find my phone line, even though the main house has a phone line and is currently being used for BT broadband. The main house has two lines, one for the main house, and the other for the Annex (both places have the same address).
I called up BT, asking them to check if the address has a phone line, also if there are two phone lines connected, but they said that they couldn't check.
There is no dial tone for my phone and we don't know the previous number of the phone or have a reference.
No company can 'just check' to see if certain houses have phonelines without a little more information.
However on Openreach's system there will be a trace of a line (even if ceased) if it belongs to them, this information can be accessed other CPs using Openreach or BT Wholesale as a provider. To a certain extent it will also show LLU lines.
This is a BT Retail customer forum and very little advice can be given for other CPs or Openreach (they will not deal with the public anyway). The forum Mods may be able to help if your looking to become a BT Retail customer and if they do be aware that the information they provide will be for BT and if you then went to another CP with it you will have to start all over again.
Having said that I would just place and order with your preferred CP and pay the installion fee as if it was a new line as you may then find that they 'find' the one serving your premises without having to install a new one.