Hello Patrick
Take a look at this NetworkLessons note that describes the contents of a BPDU. From this, you can see that a BPDU has a specific structure and includes a protocol identifier that indicates what type of STP is being used, and also includes an identifier that states what type of BPDU is being sent.
Beyond the structure of the BPDU itself, there are lower layer protocols that also indicate to a switch receiving a frame, that it is a BPDU being sent. Take a look at this cloudshark capture of a BPDU frame being sent:
(this was taken from this cloudshark capture)
The BPDU frame information shown in the note above only deals with the layer of the packet titled Spanning Tree Protocol near the bottom of the capture.
Note that this BPDU is encapsulated within a Logical Link Control (LLC) layer PDU, which contains the information in the Destination Service Access Point (DSAP) field as a Spanning Tree BPDU, with a code of 0x42. This tells the switch that the encapsulated information is an STP BPDU.
But even that LLC layer is encapsulated within an IEEE 802.3 Ethernet frame, which in turn uses a destination MAC address of 01:80:c2:00:00:00. This is a well-known destination MAC address defined by IEEE 802.3D as the Spanning Tree for Bridges destination address.
All of the above information tells the switch that this is a BPDU, and it is interpreted as such.
I hope this has been helpful!
Laz