If you don't have the driver disk, let Windows try and install the driver for it first. Then go to Device Manager to check whether the driver is installed. If you don't see a yellow exclamation mark, then test the modem to see if it works.
If Windows cannot install a proper driver for it, you'll see a yellow exclamation mark, and the modem will usually be described as a PCI Communications Device.
You may receive advice to download software to find out the model of the device. The truth is, most of this type of software relies on what is recorded by Windows itself, and make a report of it. If Windows can't install the proper driver, this type of software is not much use.
You will have to open up the case, plug out the modem PCI card, and see whether it says what brand and model it is (even then you may not find it).
If your computer is an OEM computer, and you still have the brand and model number of the computer, you may be able to find out what modem it is on the internet.