Road Runner Nest

Is there a way to identify a 18 spline transmission when its installed in the car??

The housings are the same, I guess if the vin on the case matches the cars vin and the car is an original 440 or hemi car you could assume that you had the heavy duty trans. If using the stock bellhousing, I'd remove the bottom cover and try to look up at the input shaft, maybe enough would show to let you determine whether it was 18 or 23 spline. If a one piece bellhousing I'd try to look in through the clutch fork opening.
I think the 18 spline is a larger diameter, also. I always assumed the gear teeth were cut at a shallower angle on these trans. If so, you could probably take the side cover off and inspect the gears. Hopefully you'll be able to see or feel enough through the bellhousing to make this unnecessary. I've never tried this, so let us know if it works.

Check the VIN. If it belonged to a 440 or a Hemi car, it at least started life as an 18 spline.

Don't think it's possible to see the spline count in the car.

I was looking at a 69 RR and the owner claimed to have a 18 Spline tranny in it....I decided not to buy....Thanx for the input!