Lettuce See

Is there anyway that I can differentiate lettuce from cabbage just by looking at it? May I avoid the ordeal of cooking a lettuce like a cabbage only to discover that the blasted object does not mellow like cabbage and is therefore not a cabbage? Yesterday, I spent 30 minutes trying to make a cabbage curry out of this lettuce. Lettuce stubbornly refused to get cooked. I am pretty sure that I picked this thing up from the section labeled "Cabbage" in the grocery store. Proves that even the folks who stock cabbages and lettuces everyday, cannot differentiate, leave alone lesser human beings like myself.