I'm trying to compare two camera bodies (Nikon D90, Nikon F100 (digital and film)) with respect to TTL flash power in different situation. I came to know that few Canon flashes can display the flash setting it is going to use when exposure is locked. I was wndering if it possible with one of the Nikon flashes.
This is impossible with any Nikon flash, with one exception: when shooting with automatic flash intensity control and if full power was deemed insufficient, but even then the readout is in EVs and not directly in fractions of the GN.
And, if I go a step above and try to answer your goal rather than the question you asked, assuming all other settings at their defaults, the F100 can get an extra fraction of flash power in TTL mode due to it not using preflashes.
How much this matters is debatable due to it mostly being an issue for wimpy built-in flashes (the one used in the D3100 goes from 13 to 12 meters), how the majority of wasted energy is due to the angle of the illumination cone and lens aperture, and the ability of using flash premetering ("FV lock") if the D90 supports the feature.
Will tell you everything you need to know.
EXIF data is where it is all kept but unfortunately it is not a problem that Nikon has addressed.
It basically tells you that you cannot do it. Flash shooting distance range (in TTL auto flash mode): 0.6m to 20m (2 to 65.6 ft.) (varies depending on the ISO sensitivity, zoom-head position and lens aperture in use). If you would like to read the ins and outs of the SB-600 head here, http://www.mir.com.my/rb/photography/hardwares/classics/NikonF5/Flash/SB600/index.htm
Otherwise, you may have to try and shoot in manual but there are many different types of TTL with the SB-600:
Sorry I cannot answer your question other than looking at EXIF after every shot, but I know its a pain.