I was learning Mandarin/simplified for a while and it took me some time to identify whether I should be listening in on a conversation or reading a sign or not. I don't have a good guide on Canto/Mandarin (it doesn't help that many phrases sound similar in both to a Western ear) but for writing I zeroed in on identifying the 'i' on the left side of simplified 'shuo' (说) versus the 'stack of pancakes' in traditional shuo (說). Since the 'i' appears frequently enough in characters in signs, I found it to be a pretty good signifier that what I was looking at was simplified.