You seem to be assuming that ETFs must all work like the more traditional closed-end funds, where the market price per share tends—based on supply and demand—to significantly deviate from the underlying net asset value per share. The assumption is simplistic.
What are traditionally referred to as closed-end funds (CEFs), where unit creation and redemption are very tightly controlled, have been around for a long time, and yes, they do often trade at a premium or discount to NAV because the quantity is inflexible.
Yet, what is generally meant when the label "ETF" is used (despite CEFs also being both "exchange-traded" and "funds") are those securities which are not just exchange-traded, and funds, but also typically have two specific characteristics: (a) that they are based on some published index, and (b) that a mechanism exists for shares to be created or redeemed by large market participants. These characteristics facilitate efficient pricing through arbitrage.
Essentially, when large market participants notice the price of an ETF diverging from the value of the shares held by the fund, new units of the ETF can get created or redeemed in bulk. The divergence quickly narrows as these participants buy or sell ETF units to capture the difference.
So, the persistent premium (sometimes dear) or discount (sometimes deep) one can easily witness in the CEF universe tend not to occur with the typical ETF. Much of the time, prices for ETFs will tend to be very close to their net asset value. However, it isn't always the case, so proceed with some caution anyway.
Both CEF and ETF providers generally publish information about their funds online. You will want to find out what is the underlying Net Asset Value (NAV) per share, and then you can determine if the market price trades at a premium or a discount to NAV.
Assuming little difference in an ETF's price vs. its NAV, the more interesting question to ask about an ETF then becomes whether the NAV itself is a bargain, or not. That means you'll need to be more concerned with what stocks are in the index the fund tracks, and whether those stocks are a bargain, or not, at their current prices.
i.e. The ETF is a basket, so look at each thing in the basket. Of course, most people buy ETFs because they don't want to do this kind of analysis and are happy with market average returns. Even so, sector-based ETFs are often used by traders to buy (or sell) entire sectors that may be undervalued (or overvalued).
