The movie explains that Cobb adopted Mol's totem after her death. Do we ever know what his totem was before he adopted her spinning top?
Short answer : we are never told explicitly what was Cobb's totem before the spinning top. and IIRC, he doesn't say at all it is his totem:
This one was hers. She'd spin it in a dream and it would never topple.
Just spin and spin...
Now, if you are ready to dig into The Internet, you are going to find a lot of different theories, which is one of the critical successes of the film (and I would say is characteristic of Nolan's work) - make your own opinion.
I will add that the most accepted theory is that his totem is his wedding ring. Did you notice how hard it is throughout the movie to look at Cobb's left hand? Why would he hide his hand like that if it wasn't to hide his true totem, and so, his true weakness?
tl;dr - He had no totem before he started using Mal's spinning top.
After some consideration on the matter, I've come to the tentative conclusion that Cobb didn't have a totem prior to Mal's death and that her spinning metal top (a childhood toy) wasn't originally a totem but simply a childhood toy of Mal's that Cobb took possession of after her death and subsequently used as his personal totem.
So how did Cobb and Mal determine that they were in reality or still dreaming prior to her death?
Mal and Cobb were engaged in experimentation with the PASIV machine and doing things that hadn't been done before. Before Cobb and Mal got stuck in Limbo, it seems that no-one had ever gone deep enough into a dream-within-a-dream to significantly confuse reality with the dream world and hence to need some way of find out what the situation was (i.e. to need a totem).
COBB: We were on a job. Exploring dreams within dreams. But we didn’t understand how your mind can turn hours into years. How you can get trapped. Trapped so deep that when you wash up on the shore of your subconscious…
INSERT CUT: MAL LIES ON THE SAND, STARING UP AT A CLOUDLESS SKY, WAVES WASHING OVER HER…
COBB: You can lose track of what’s real.
ARIADNE: How long were you stuck?
Cobb pauses before he answers. Looks at Ariadne.
COBB: Fifty years.
So what about his wedding ring? Couldn't that be Cobb's totem?
We see Cobb him wearing his ring in both dreams (limbo) and reality and hence it would be no use in determining which state he was in. Myth busted.