The Physical Coding Sublayer is responsible for delimiting the frames, and sending them up to the MAC layer.
 
In Gigabit Ethernet, for example, the 8B/10B encoding scheme uses a 10 bit codegroup to encode an 8-bit byte. The extra two bits tell whether a byte is control information or data. Control information can be Configuration, Start_of_packet, End_of_packet, IDLE, Carrier_extend, Error_propagation.
 
That is how a NIC knows where a frame start and ends. This also means that the length of the frame is not known before it has fully decoded, analogous to a NULL-terminated string in C.

The control symbols within 8b/10b are 10b symbols that are valid sequences of bits (no more than six 1s or 0s) but do not have a corresponding 8b data byte.

The end of a frame is signaled by loss of carrier or by a special symbol or sequence in the line coding scheme for a particular Ethernet physical layer, so the length of the frame does not always need to be encoded as a value in the Ethernet frame.