I don't think your general guideline of 50% certain is accurate. When you make an accusation is more dependent upon how much your opponents know, and how likely you are going to be able to make another useful suspicion before the end of the game. BTW, there aren't that may possibilities for certainty at which you would make an accusation:
100% - Deduced all cards
50% - Know all cards except 2 of one type.
33% - Know all cards except 3 of one type.
25% - Know all cards except 2 of one type, and 2 of another (or 4 of one type).
16.7% - Know all cards except 3 of one type, and 2 of another (or 5 of one type).
... doubtful you would make an accusation at this point.
I have played quite a few games, and I am pretty sure that in most of them, a player that won made an accusation before they where 100% sure. It is very difficult to be so far ahead that only you know will be 100% sure before everyone else knows enough information to be 100% themselves, or close enough that they shouldn't risk a guess before you get your next turn. I will give some concrete examples below for illustration (when I can think of them).
How can I tell if other players are close to making an accusation?
There are multiple ways to determine what your opponents know (or should know).
A suspicion does not get disproved. If a suspicion goes all the way around the table, there are only two possibilities. The player making the suspicion has none of those cards in their hand, in which case that player is 100% sure and should make an immediate accusation and win the game. The player player making the suspicion has one or more of those cards in hand.
A suspicion for a particular Suspect, Weapon, or Room is not disproved by a player that previously disproved a suspicion with that element.
Imagine that you suspect: Col. Mustard, with the Candlestick, in the Billiard Room, and Player A immediately to your left disproved them.
At a later point, you suspect: Col. Mustard, with the Candlestick, in the Conservatory, and Player A cannot disprove them.
Your opponents should know, even without seeing Player A's cards that Player A has Billiard Room in their hand (and doesn't have Col. Mustard or Candlestick).
- A player continually uses the same Suspect, Weapon, or Room. While this cannot be directly deduced from the process of elimination as in the above examples, it is highly likely that the player that is continually reusing the same Suspect, Weapon, or Room is their suspicions because, they are trying to make a suspicion that will go all the way around that will tell them with certainty one or more of the elements that they don't know. You know that either: 1) They have that card (or multiple cards) that they keep reusing, or 2) They don't have any cards of that type (Suspect, Weapon, or Room), and are using the card in the envelope because they have no choice.
I wish I could provide some concrete examples to illustrate when you might venture a guess at 50%, 33%, ..., but it has been ages since I played a game of Clue. It is difficult to envision a situation that doesn't sound contrived without playing a real game, and using a real world example in which you almost sure of the answer, and you know exactly which cards your opponents still think might be in the envelope. I might try to convince the family to play a game of Clue, so that I can edit the post to be more helpful.