The easiest way is to do a teststream on a testaccount as dping mentioned. For Twitch there is the option to append something like "?bandwithtest" behind the streamkey so twitch knows you only want to test and doesnt put you live, but other services might not have a similar feature. Hence, noone wrote a bandwidthtester for them.
Also, the connection can change or the server might run into problems, and all tests will be for nothing. In the end, as always, I would recommend to stay in reasonable bitrate ranges that fit to your available upload speed. Do some tests and find the, at that time, most stable server.
To see if OBS is generally configured without obvious problems you can use our log analyzer, which will also tell you if your bitrate is too low for the current resolution settings etc.