Looking at the Ethernet entry on Wikipedia, I can't figure out how it's indicated how long the Ethernet frame is. The EtherType/Length header field apparently can indicate either a frame type or an explicit length, and I'm guessing that in the case of a frame type, it has to do some other logic to figure out how long the packet is. For example, if the EtherType field is 0x0800, that indicates an IPv4 payload, and so the receiving NIC would have to examine the first 32 bits of the payload to find the length of the IP packet, and therefore to figure out the total length of the Ethernet frame, and know when to look for the end-of-frame checksum and interframe gap.
Does this sound correct? I also looked at the IEEE 802.3 spec for Ethernet (part 1, anyway) which seems to corroborate this, but it's pretty opaque.