Pretty straightforward.
On NZG click on Available Now and it displays girls who have activated that feature to advise they are working and available now.
On Escortify they have a boost function so the girls at the top of the VIP section, or down below in the Regular section display a time when they boosted. The most recent is the first ad, superseded when someone else boosts after them. The implication, if they are online and they have boosted their ad they are available and looking for work.
Of course the other indicator is look at the posted available hours, not always accurate, but text and see if you get a response.