Detection of potassium ions
What do you want?


If you're just looking for a source of potassium ions you could just buy $\ce{KCl}$ or some other compound, they're relatively cheap if not pro analysis. If you actually want to test for $\ce{K+}$:


What else do you know about the product?


Tests are completely different for different amounts of $\ce{K+}$. Does the vendor specifies the amount of each compound on the product's label?


How sure do you need to be?


Do you need to do a QUALITATIVE analysis or a QUANTITATIVE analysis? If you need to be absolutely sure about the concentration then you need to guess it's ridiculously low - you'll probably need at least AAS (Atomic Absorption Spectroscopy) equipment, which can't be accessed by anyone nor find anywhere, but you can order an analysis. Alternatively, you can use an electrode - keep reading .

Some ways of testing that I can think of:

(Of course you'll need to dissolve the powder in at least distilled water to do all the following tests, and do a blank with the same water - the more pure the water the more accurate your results.)


For big amounts of $\ce{K+}$


The flame test suggested by Paul J. Gans on the comments above could work, although I never actually did it.

Another way is doing the following titration:

$$\ce{KCl(aq) + AgNO3(aq) -&gt; AgCl(s) + KNO3(aq)}$$

$\ce{AgNO3}$ can be found at any store that sells chemicals. We used 0.1 M for small amounts of $\ce{Cl-}$.

What you are actually testing for is $\ce{Cl-}$ ions, but if the amount of K = the amount of $\ce{Cl}$ the job is done. Now, since the powder contains potassium citrate this is just a qualitative test, although it can be very accurate for small concentrations if properly executed (see Mohr method). Silver chloride is a white solid that is barely visible in small amounts, and a similar  titration's end point is just a few drops (Mohr method), and there are a lot of possible interference so you should not expect precision unless you do some pro work.


For small amounts of $\ce{K+}$


I think the best way of doing this is using a potassium-selective electrode. I never used one of those, but other ion-selective electrodes work great on water analysis, detection levels were a few mg/L on our old ones!

http://en.wikipedia.org/wiki/Potassium_selective_electrode

And of course, you can use AAS.