That’s one type of smurfing that rose with F2P. You create an account that you rank in bronze, and then you climb up the ladder in unranked. More information in the Q&A thread if you’re interested about the smurfing part.
While some hackers do also smurfs, they still are a very rare occurrence, and it’s much more difficult to prove the hack than the smurfing.
It’s not if someone claims to be hacking that they really do, and your opponent could’ve perfectly been referring to having cheated the matchmaking (which he did to get placed as a bronze player). Most real hackers I’ve met stopped chatting when I pointed out their suspicious behavior. Admittedly, I may have met less than 10 in my whole career.
To find out, you have to analyze the replay, see if the player reacts to something he does not see (specially to correct a previously given order, and even more to moving elements), see if he’s watching into the fog of war, that kind of thing. A build order that doesn’t look like anything known can be a hint, as it suggest the player is used to adapt it on the fly. Not scouting also is a clue. Abnormally high APM spikes (800-1000) can also pinpoint some kind of hacks. There might be other players of your MMR who have also played this user (specially if you’re around master and above), and so the suspicions can be shared.
Once you have found convincing enough evidence :
- You upload the replay on a site such as drop.sc, Replaystats, or any filehoster.
https://drop.sc/ - Then you report the player (right click on his profile → report → hacking), and you paste the link to your replay, with a list/description of the suspicious moments.
Sometimes it might look like the team in charge of hacking doesn’t reacts, but they do always check the replays, and can in fact be trying to observe the user in order to shut the hack instead of banning the hacker right away. And sometimes the hacker might just get another update on his hack. It’s a kind of arms race, in a way. 