Never had or used one as I was very, very fortunate to learn about kites and wind from Mr. Ray Bethell. He also stressed the gear...so I always take enough kites and line sets to cover all bases as conditions can change in an instant. You must also know your kite(s) and line(s) well enough to make the appropriate choice(s) and this comes with a lot of time (and loot) invested...but what a great way to spend time (and loot)!!
I live right across the street from the ocean so obviously the waves will let me know how windy or not it is. Now don`t go "oooh those lab grade ocean winds" as they certainly are not!!!
Things also get to the point in wherever you are...you are aware of the wind and imagine what kite you would be flying...
How to judge wind speed? Built in wind meter called the hair on my neck! Or is it my ears? I don't think I ever owned a meter and don't now. The more you fly, the more you will be able to "fit" a kite selection to the conditions. For me, the worst that can happen is I need to change kites. Already got the lines out and that is the biggest part. Just go with the flow.....!
But Rob has the right idea if you really want/need a meter. By that second battery, you should be able to guess, pretty much.