One of the biggest clues on telling if a FY cel is 
from the OAV or TV, is the OAV's sharper design. 
The animation in TV is a bit more flowy-like, 
while the OAV tends to be more crisp. This is 
especially noticeable in Taka/Tamahome. Tasuki is 
easy to tell because he wears a completely 
different outfit in the OAV (a purplish/red 
outfit), while he wears his classic blue & black 
outfit in the TV series. The one exception is the 
OAVI, but I haven't seen but a few cels from this 
part of the series.

Chichiri is probably the hardest to tell. ^^; You 
probably need to actually see the OAVs in order to 
find out if the cel is from them or not, as he's 
wearing the same outfit always. You could ask 
someone here if you have a question about a FY 
cel, or you could e-mail me and we'd be glad to 
identify a cel for you. ^^

Hope this helps!

Ryouko