The expression browser is Prometheus' built-in interface for running PromQL queries. To use the expression browser, navigate to http://<machine-ip>:9090/ and choose the "Table" view. It should look like this:

The "Table" tab shows the most recent value of each output series of an expression, while the "Graph" tab plots values over time. The latter can be expensive (for both the server and the browser). It is in general a good idea to try potentially expensive expressions in the "Table" tab first and switch to the "Graph" tab once you have sufficiently narrowed down the results of an expression. Take a bit of time to play with the expression browser.

Suggestions for expressions:

This shows you the total number of samples ingested into Prometheus' local storage since process start time:

prometheus_tsdb_head_samples_appended_total

This expression shows you the number of samples ingested per second as averaged over a 1-minute window:

rate(prometheus_tsdb_head_samples_appended_total[1m])

This shows the value of the synthetic up metric that Prometheus records for every target scrape, in this case, scoped to just the prometheus job:

up{job="prometheus"}

The value for this metric is set to 0 whenever a scrape fails and to 1 when it succeeds.

Try viewing all expressions in the "Table" view as well as the "Graph" view.