How to tell which package is defining \ifpdf?

Question

I'm experiencing a problem similar to this one: Package ifpdf Error

After upgrading to a more recent version of TeX Live, my pre-existing documents stopped compiling, with complaints that \ifpdf was already defined. My project is large and pulls in a lot of packages, so I don't know which one is defining \ifpdf. Using \let\ifpdf\relax after the \documentclass, as suggested in the answers to the other question, fixes the problem on the newer TeX Live. However, this breaks the document on older versions of TeX Live, where I still need it to compile for various reasons. (I think I'm getting malformed PDFs from pdfTeX 3.1415926-2.4-1.40.13, but that's another topic.)

Is there any simple, straightforward way of finding out which package is defining \ifpdf when it shouldn't be? I could presumably figure it out by trial and error, by bisection, but this is a large, complicated project, and that would be a lot of work. If that worked, would knowing this allow me to make my file compile on both versions of TeX Live?

Is there some other way of making a more robust fix? Can I somehow test somewhere to see whether \ifpdf has been defined, and then define it conditionally at that point?

[EDIT] I figured out the problem. It wasn't another package, it was some code in my own .cls file, which I'd cut and pasted from somewhere probably 15 years ago:

\newif\ifpdf
\ifx\pdfoutput\undefined
\pdffalse % we are not running PDFLaTeX
\else
\pdfoutput=1 % we are running PDFLaTeX
\pdftrue
\fi

\ifpdf
\usepackage[pdftex]{graphicx}
\else
\usepackage{graphicx}
\fi
\AtBeginDocument{
        \ifpdf
        \DeclareGraphicsExtensions{.pdf,.jpg,.png}
        \else
        \DeclareGraphicsExtensions{.eps,.jpg,.png}
        \fi
}

I was able to make it run on both (modern) versions of texlive by shortening this to the following:

\usepackage{graphicx}
\AtBeginDocument{
        \DeclareGraphicsExtensions{.pdf,.jpg,.png}
}

This probably breaks compatibility with dvi-flavored tex, but I don't care about that.