IF they wanted it to know themselves, yes. If they wanted to hide it from other people, no. There are some ways to detect if code was changed, but there are steps you can take to erase those too. Like investigating a murder, they could check for finger prints, DNA, security footage, etc, but if the murderer thought of those things they could pre-emptively counter them doing things like wearing gloves.

The only real hope would be If they were dumb enough to use third party tools or code which may have their own logs that could point to changes likely being made.