Who says you can't??? Lol
There are various sources I have come across saying that ISOLATED subendocardial ischemia rarely exist on ECG, and some even saying that it doesn't exist on ECG. One of them is by Dr. Smith (well know for ECG interpretations):
"But stress testing shows us that ST depression
does not localize. In other words, when the stress test shows ST depression in an apparent distribution, it does not correlate with the echo, nucleide imaging, or cath. Thus, subendocardial ischemia, for an unknown reason, does not localize. When there is ACS with ST depression
due to subendocardial ischemia, it is diffuse (II, III, aVF, V4-V6). If it is focal, one should suspect that it is reciprocal ST depression, reciprocal to ST elevation elsewhere."
You can see GLOBAL subendo, or transmural ischemia on ECG and may be "subtle T wave changes" for localized "big enough to generate vector current" subendo (I think).
Sorry it didn't let me post a direct link, but feel free to google that statement