The most common sign that a fruit machine is likely to payout is when one or more players have repeatedly put money into it, but had no major payouts. This is a sign that it is likely (but not definitely) about to payout, and you'll often find pub fruit machines being played in this way.
A standard slot machine can typically hold around 600 to 800 quarters when filled to full capacity. The exact number can vary depending on the specific model and design of the slot machine.