You don't need the small levels, they won't tell you the whole picture. You need either a 6' or ideally a 10' straightedge (I use an aluminium 1x3 that's 10' long)
Put it down anywhere you want to start - Direction doesn't matter. If the bar rests completely flat, then that section is good. Check in a circle, keeping the centre roughly the same. Then check along walls and through doorways etc, until you feel you've covered enough of the area to find all the spots.
If you notice a gap at any time between the bar and the floor, stop and measure that gap. If its less than 1/8th you're still OK. Over 1/8th and you've found a problem, either a high on one or both sides of the bar, or a low in the middle. A gap doesn't always mean its a low spot, if one side of the bar is on a high spot it will create visible gap that can be quickly mistaken for a low spot.
To determine how high a high spot is using the straight edge method, put the middle of the bar over the suspected high spot - you should be able to rock the bar back and forth like a see-saw. Let the bar come to rest and measure the gap at the end of the bar (it's important that the high is in the middle of the bar for this measurement, as you can manipulate the bar to produce a higher gap by sliding it over the point in different ways - I fight with the concrete levellers over this distinction all the time)
Once you have the areas mapped out, pull the floor and fix the subfloor, then reinstall. Side note, some installers will cheat a little to even out a subfloor if you have an underlayment (cut the underlay around a high spot, or double up a low spot) or are gluing the product down (excess glue fills the low spots)
And remember, LEVEL doesn't matter, FLAT is what's important. Hope this helps!