Basically you can't unless you get fixed odds. However, you can tilt things in your favour by betting when the win pools are large..E.G. Saturday's, Wednesday's.

Also look at the trends, often in the last 5 minutes horses that are in the top three or four betting wise will have a good chance of trimming up and others drifting in price.

One way of further ensuring you get the right price is betting with a bookmaker who will give you top fluctuation.

If you are only betting on the tote, then look up the prices in the state pool of the race.
For example..If you have a bet in Perth, go to the Perth tab website and look at the price there. Chances are if there's a big difference in that price and your State price, it will go close to the home state price...but not always.
It's just a guide.

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Treat your selection and staking methods not with optimism, nor with pessimism, but with realism.