What is the best evidence that a glacial erratic has been transported?

The best evidence that a glacial erratic has been transported is its composition differing from the local bedrock, indicating it originated from a distant location. Additionally, the erratic often exhibits striations or scratches on its surface, caused by the movement of the glacier over it. The presence of these features, along with the erratic's location in a landscape shaped by glacial activity, strongly supports the conclusion that it was transported by ice.