I do not know how to tell (for sure) the difference without firing but I do know this.
The S&W 99 is DA and SA but has different modes of function.
When the striker is in the decocked position, the first shot is fired with a traditional double action pull with subsequent shots fired in the single action mode.
The gun can also be fired from a long single action trigger position. When the slide is used to chamber a round, the striker can be decocked, or left in this cocked condition with the trigger fully forward, resulting in a first-shot single action trigger pull with a lot of take-up.
The 99’s trigger also has a double strike capability. If a round doesn’t fire on the first strike of the firing pin, the 99 allows another quick trigger pull without working the slide.
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