Unmarked ESC

I have an ESC which is a plain black heatshrink covered item. Is there a way of finding out its rated Amps? Apart from loading it until it blows up.

Size/weight of an ESC is directly proportional to its current rating. Check the weights of a few different ESCs online and then compare to work out your approximate rating.

Also, most of us generally use a bigger ESC than we need. If the system draws 30A we will use a 40A ESC, for example. But most ESCs can take a lot more abuse than that.

Finally, ESCs are relatively cheap and if you do burn your unbranded controller it won't cost much to replace it in the future.