Visit any racetrack in the country, and you will find one or more tote boards scattered about the premises. Some may be elaborate, other quite plain, but they all serve the same function. Listed on the tote board you will find the program number for each horse in the race plus the current odds on the horse. Some will also show the morning line odds and the amount of money currently in the Win, Place, and Show pools.
The odds on the tote board change at different intervals at different racetracks, typically every thirty to ninety seconds in my experience. It usually depends on how much money is handled by the track in question. The more money wagered on any one horse the lower its odds will be. The less money bet on any one horse the higher its odds will be.
To determine ahead of time what your potential win payout will be is relatively easy. A horse listed at odds of 1/5 will payout $1 for every $5 wagered on it. So, if you bet $5 on a horse with those odds, you will receive $6 in return — the $5 you bet plus $1 — assuming, of course, your horse is the winner of the race. If you are a $2 bettor, your return will be $2.40.
Not all payouts are as easily determined though. If you wager on a horse with odds of 5/2, you will receive a minimum payout of $5 for every $2 you bet, but it is entirely possible the odds on your horse are actually 2.8 to 1 instead of 2.5 to 1, meaning your payout will be $7.60 for every $2 you wagered. It appears that converting to decimal odds is not a high priority in the racing industry despite the fact the technology to do so has long existed. After all, why would they want to make things easier for their patrons?