How do you determine a good etf/index?

I see people all the time saying you should invest more into indexes and etfs rather than individual stocks, but it seems so much easier to determine if an individual stock is a good value or not.

I've been looking through a bunch of etfs lately but they almost always have some stocks that look bad to own and I also find it to be impossible to know what's a “good time” to buy into an etf, like how am I supposed to know if $55 is a low or high price on a group of stocks that all have different weight?

I've had some money in a total market etf, but for more specific ones I feel like I have absolutely no clue what's a good value.