How do you determine which ethernet standard I am using?
The standards for modern wired ethernet are defined in 10BASE-T, 100BASE-TX and 1000BASE-T see Wikipedia.  These are defined in various IEEE standards in the 802.3 series.

Twisted-pair Ethernet is point-to-point.  Bigger networks are built up using switches and routers.  As such, the devices on each end of each link negotiate between themselves what is the fastest speed they both support - typically 10, 100 or 1000Mbps.  It's quite possible to have a network where different devices are talking at different speeds.

This speed is totally unrelated to your ISP's broadband speed, which is probably not delivered over Ethernet anyway.

I can't imagine that building your own CAT-5 Ethernet cable is going to be easy.  The standards are quite strict.  Through something approximating to the standard may work over short distances.