On each exchange, users buy/sell cryptocurrencies (that's what makes them exchanges). Users can sell/buy them using fiat currencies (e.g., USD), other cryptocurrencies, or anything else the exchange accepts (including, but not limited to, vouchers redeemable only by the exchange, gold, and spotted owls).
In the case of a currency pair such as ETH/USD, at a particular moment, there might be a bid for (an offer to buy) 500 ETH for $500 ($1/ETH) and an ask for (an offer to sell) 10 ETH for $11 ($1.10/ETH); this is known as an auction market and is, I believe, the dominant cryptocurrency exchange type (cf. shapeshift which is a dealer market). The difference between the bid and ask prices (spot price) is known as the spread. The smaller the spread, the better the pricing information (IMHO) since the price of a trade lies anywhere between the spread. Some exchanges may list some blend between the bid and ask price as the currency's list price. Others may use the last price a trade was made at.
The notion of a "true price" is a lie, just like the notion of a currency exchange rate. If you watch the news and see a currency price for a particular day, at least in places I'm familiar with, what you're usually seeing is the last trade according to one time source that occurred before a particular time.
Instead, what you may be interested in is how much it would cost you to buy an ether with USD or how much you could sell an ether for. What you probably want to do in that case is look at the different fees for withdrawing/depositing. For example, if a particular exchange charged 1% on deposits and withdrawals, it might make more sense in your situation to add 2% to the posted rate if you're looking at buying or subtracting 2% from the posted rate if you're looking to sell.
If an exchange is seen as being unable to meet its debt obligations, you would also need to take that into account. For example, if an exchange were hacked and ether was stolen, you might see very cheap ether on the site: users who are owed ether by the exchange might be in doubt as to whether they could withdraw their ether and, instead, settle for fewer USD/CAD/GBP/whatever than they might ostensibly obtain elsewhere. Similarly, in countries that have capital restrictions, currency that is seen as superior to the native currency might fetch a better through unofficial channels than through official ones (e.g., USD in Venezuela).
Exchanges that work using USDT instead of USD should also not have their USDT valued on par with USD. Firstly, there is the risk that Tether may be cooking their books. Secondly, Tether charges a fee if you decide you want to redeem their tokens for USD.