https://www.bbc.com/news/world-africa-37243190

Too many voters

Watch the turnout figures ‒ they can be a big giveaway.

You never get a 98% or 99% turnout in an honest election. You just don't.

Voting is compulsory in Gabon, but it is not enforced; even in Australia where it is enforced, where you can vote by post or online and can be fined for not voting, turnout only reaches 90-95%.

The main reason that a full turnout is practically impossible is that electoral registers, even if they are recently compiled, can rarely be 100% up-to-date. Even if no-one gets sick or has to travel, people still die. And when a register is updated, new voters are keen to add themselves to the list.

No-one, however, has any great enthusiasm for removing the names of those who have died, and over time the number of these non-existent voters increases.

A high turnout in specific areas

Even where the turnout is within the bounds of possibility, if the figure is wildly different from the turnout elsewhere, it serves as a warning.

Why would one particular area, or one individual polling station, have a 90% turnout, while most other areas register less than 70%?

Something strange is almost certainly going on, especially if the high turnout is an area which favours one particular candidate or party over another.

Large numbers of invalid votes

Keep an eye on the number of votes excluded as invalid. Even in countries with low literacy rates this isn't normally above 5%.

High numbers of invalid votes can mean that officials are disqualifying ballots for the slightest imperfection, even when the voter's intention is perfectly clear, in an attempt to depress votes for their opponents.

Delay in announcing results

Finally something that is not necessarily a sign of rigging, but it is often assumed to be so.

Election commissions, particularly in Africa, can appear to take an inordinately long time to publish official results.