The 1990 CompuServe GIF89a specification defines a "graphic control extension" like so:
The Graphic Control Extension contains parameters used when processing a graphic rendering block. The scope of this extension is the first graphic rendering block to follow. The extension contains only one data sub-block.
This block is OPTIONAL; at most one Graphic Control Extension may precede a graphic rendering block. This is the only limit to the number of Graphic Control Extensions that may be contained in a Data Stream.
As this block is optional, how does one determine whether or not an arbitrary GIF image contains a graphics control extension?
As it turns out, the specification pretty plainly states that all extensions, including the Graphic Control, Plain Text, Comment, and Application extensions, are demarcated by the presence of a 0x21 at the start of the block.
What threw me off is that GIFs contain a flag located in the Logical Screen Descriptor that denotes the presence of an optional Global Color Table block, but there is no such flag to denote the presence of a graphic control extension (not sure why). It either comes right after the Logical Screen Descriptor or it doesn't. Hope this helps someone down the road...