When a pendulum regulated clock is running well, you will hear it tick. The loudness of the ticking will vary among clocks, but a clock in beat will produce a even "tick tock" sound. The "tick" and the "tock" will have an even rhythm between them. One will not sound longer than the other. A clock that is not in beat will have an uneven rhythm in the the ticking and will run no longer than 10 or 15 minutes.
To put a clock in beat, you can have a clockmaker come to your house to adjust either the pallets or the crutch. However, he will charge a service fee. Another solution is for you to try to level the clock to where it is in beat. This is possible with a wall or mantel clock, but a grandfather clock is going to require the attention of the clockmaker.