Went to adjust the choke on my 2150 and realized the choke cover does not have an index mark on it anywhere. I do have marks on the carb side, but the black cover is without...

Carb came from National Carburetors with the choke thermostat housing cap already installed. I looked at my old carb choke housing and it too is missing an index mark.

Is there a way to determine where the mark should be? It's a hot air only type - no electric assist.

 

I wouldn't be concerned about no marks.
With the engine cold, turn the cap until the choke plate closes, then turn just a little more to put a very slight load on the choke plate.
Start the truck and if the choke comes off (opens) too quickly, adjust the tension a little more. If the choke stays closed too long, back off on the adjustment.
This may take a few tries but it's not difficult.

 

A little white paint on a piece of card board. And a paper clip. Make a line on the choke cap with the paper clip dipped into the white paint went you have it set.

 

If your choke is working properly, it's going to open a bit on startup, because you have a vacuum operated choke pull-off and it's job is to crack the choke a little once the engine is started so it can get a little air.

Don't worry about the marks. As mike said, with the engine cold, open the throttle part way and, while holding the throttle open, turn the choke coil until the choke just closes. Then let go of the throttle and tighten the choke coil screws. That's all there is to it. You can test it by opening the throttle, pulling the choke plate open, and releasing the throttle. Then open the throttle again and see if the choke plate closes all the way. If it does, you are done.

The reason for opening and closing the throttle as I suggested is that your choke is tied to the throttle linkage via the high-idle cam that jacks up the idle speed when the choke is deployed. You must open the throttle to allow the choke to move freely.

 

If you are that worried about having a mark, carefully file a fine score into the edge of the choke coil housing and another matching mark where it lines up with a metal parts.
Fill the score with silver paint if you want to make it all purty.