Was it that known among uber fans at the time that it was Maggie? My impression at the time was that people that got it right simply were guessing every character in the series.
It was "known" in terms of it being logically deduced thanks to all of the clues left within Who Shot Mr. Burns, Part 1 - some of which were denoted by Sideshow Mel in the premiere and some that weren't.
The sundial...
Referenced in both the premiere and the alternate ending that was recorded and included in The Simpsons 138th Episode Spectacular clip show was the immediate clue. MS or WS could only pinpoint a handful of suspects - Waylon Smithers, Marge Simpson, Moe Szyslak, Groundskeeper Willie, Sideshow Mel and Maggie Simpson. Mona would have also satisfied the initials but she wasn't introduced until after this two-parter.
Marge was innocent due to being the one character on-screen when the shot was fired.
An awful lot of time was used to set up Smithers with an alibi... his Pardon My Zinger line was backed up with this animated ad on Moe's TV (which is seldom animated except for an obvious gag) which included a time that would have contrasted with the time of death.
Moe's last name wasn't known until this episode but the fact that it was called out on his bartender's license in this shot simply reinforced the importance of the sundial clue.
THIS shot from the finale...
Which clearly showed that Burns had been killed by his own gun thanks to his visibly empty holster. Hence making all other guns shown being stroked at the meeting or missing (such as the missing gun that was buried in the Simpsons backyard) blatant red herrings.
The audible sounds of a lengthy struggle before the shot. Burns can't struggle with anyone at length (except with a baby) due to his feeble strength. Maggie became a possible suspect due to having been separated from Marge immediately prior to Marge's defined alibi due to being on-screen when the shot was fired. And there was that "stealing candy from a baby" line earlier in the episode. Maggie was shown holding a sucker in this scene.
The clue that sealed it for me when this episode was combed through with a fine-toothed comb on alt.tv.simpsons
PRIOR to the premiere was this revelation.
In the scene when Mr. Burns uttered, "You all talk big, but which of you has the guts to stop me?" everyone's eyes were animated to waver and glance away... except for Maggie. She never does.
And lastly... just to reinforce that Usenet deduced the outcome a full three months prior to the actual reveal, scope out this link.
When alt.tv.simpsons predicted Maggie shot Mr. Burns (June 2, 1995).
In 1995, for the season finale, Mr. Burns was shot, and we were supposed to spend all Summer wondering who shot him. The newsgroup posters immediately went to work, and on June 2, 1995 a Harvard student (or the very least, someone with a Harvard email), Gary Chase posted his theory on how Maggie shot Mr. Burns:
From: nh…@gauss.med.harvard.edu(Gary Chase 732-5133)
Newsgroups: alt.tv.simpsons
Subject: The Maggie Theory: A summary
Date: 2 Jun 1995 18:51:13 GMT
Message-ID: <3qnmj1$k…@netope.harvard.edu>
While nothing is certain, it seems that Maggie has become the
“prime suspect” of many of the posters to ATS. The following
is a summary of clues/evidence/stuff….
- Burns speaks to the “attacker” as if he is talking to a
child or animal.
- Maggie was left alone in the car with the door open and
not buckled into her car seat.
- She was last seen in the car holding a large sucker. Burns
had earlier discussed wanting to steal candy from a baby.
- All of Burns comments to his “attacker” are consistant with the
Maggie theory. Specifically: He struggles with that person over
some object, most likely the sucker. Only a baby (or a dog) would
allow a “struggle” since any other person would quickly over-power
the notoriously weak Burns. The attacker is someone Burns knows; he
knows Maggie. The attacker doesn’t speak; Maggie never speaks.
Burns says, “Get your HANDS off me” thus eliminating the Santas
Little Helper theory.
- Burns wound only went through his shirt not his jacket, suggesting
he was leaning forward, with his Jacket open, when shot.
- Only Maggie (and an unidentified black male in the back:Carl maybe?)
doesn’t avert her eyes when Burns says “Who will stand up to me?”
- Maggie fits the MS or WS sundial “clue”.
Thats about it. Please add other clues that I have missed.
To be fair - because I was there - Gary was merely summarizing everything. We had people deduce Maggie as the killer
within a week.
Entertainment Weekly was keeping tabs on the newsgroup discussion and hashed out all of the clues prior to the season premiere in
this article.