The dollar exchange rate varies a lot and randomly, so it is impossible to predict whether the dollar is cheap or expensive.
However, we can use some simple tools to get an approximate understanding of the value of the dollar, although not entirely accurate.
A good and free tool is the Focus newsletter from the Central Bank.
The bulletin shows weekly market expectations about the dollar for the end of the current year and subsequent years.
You can access and subscribe to receive the bulletin by email on the Central Bank website: https://www.bcb.gov.br/controleinflacao/inscricao_focus. Another practical way is to follow the Central Bank's social networks, as the institution posts bulletins on these networks.
Another useful tool is to understand the range of dollar values in the last 30 days.
This way, you can get an idea about the values that are in the recent memory of the market.
You can easily look up this information on Google Finance or another market data provider on the internet.
For example, see below the query we made on March 10, 2023. The dollar was at BRL 5.20. This value is relatively high considering the last 30 days, although, in that same period, the dollar has traded in some days in values greater than R$ 5.20.
Thinking about market expectations for 2023, the value of BRL 5.20 is not high, which suggests that it makes sense to buy the US currency at this price level.
Lastly, another important tool is to split currency purchases over time, especially when you don't have a dated commitment with dollars.
In this case, the investor can wait for the best moment to convert his reais into dollars, splitting his purchases from time to time.