Unless you take your turtle to the vet and do some testing (commonly done by taking a feces sample in for analyzing), there is no clear way to determine if your turtle has salmonella. Salmonella has a pretty low chance of dissapearing as a turtle ages, but know this:
90% of all turtles, reptiles, or even other animals (chickens for example) contain salmonella, it's a very common occurence in pets, so don't be alarmed if your turtle does have salmonella, i'm pretty sure most of the people here have their turtles with salmonella, and don't do any testing unless they have a fat wallet,

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Most of us don't check our turtles for salmonella, it's pretty much what comes with owning a turtle (or any other pet). For many people that come in contact with salmonella, there will be no reaction, or some small side effects such as diarrhea, only a small percentage of the population will experience serious symptoms from exposure.
Salmonella most commonly exists in the insides of a turtle, and the best way to reduce or eliminate the release of large amounts of this bacteria is to simply make sure that your turtle is healthy and lives in a clean environment.
The important thing to remember is wash your hands under warm water and using a good hand sanitizer such as antibacterial hand soap after handling turtles or any other pets, it's the simplest way to protect yourself.