Giving Tzedaka

There are people who go around shuls collecting but I'm not entirely sure of their authenticity if I give them money does that count as tzedaka even if it turns out they aren't legitimate?

Shalom! 

Thank you for your question. 

Let me begin with a few words about the mitzva of tzedaka, charity. 

It is both a positive mitzva of the Torah to give tzedaka as well as a negative mitzva not to refuse those who ask for tzedaka. (Deuteronomy 15:7). 

Nevertheless, this does not mean that there is always an automatic obligation to give tzedakah at any time anyone asks, since nowadays many poor people are not considered halachically poor. However, if a person presents a certificate from a rabbi etc testifying he is poor then one is should give him at least a "perutah" (around 13 Agurot). there are other Halachot regarding giving charity. 

For example, although there is a mitzva to give enough charity that will cover all the needs of the poor person, however since he is going around asking everyone for tzedaka it can be assumed that he will eventually get what they need. 

So too, one is not required to perform the mitzva of tzedakah if one is currently engaged in another mitzva. So, for example, if you were in the middle of praying the Pesukey De'zimrah and giving charity would disturb your concentration, then you would be exempt from giving charity. However if let's say you prepare the money beforehand to give later or you know that it would not disturb your concentration then one should give the Tzedakah.

Most importantly, if one is unsure about the honesty or integrity of the person requesting tzedakah then one is not obligated to give them.