I use a cracking piece of kit, a must have for any batty person! It's simply a bat detector which tunes in to the bat's echolocation/ultrasound and turns it into a sound that we can hear, that of course we cannot otherwise hear. Each species of bat has a specific frequency range. It takes time to practice using such a piece if kit and to get to know bat species through their frequencies. The frequency range on the detectors are usually 15-130kHz. There are resources where you can get a whole wealth of information about bats, including the frequency.....
The Field Studies Council produce great fold out charts for identifying anything out in the field. I have this one for bats to help with the frequency calls and the general 'jizz' of a bat, meaning their flight patterns, behaviour and size.
In the UK there are also county Bat Groups whereby enthusiasts and certified people are able to survey bat roosts. Bat roosts are typically in old buildings, churches, rooves, cellars, outbuildings, caves and trees and the bats are protected by law.
In my garden we usually observe Common pipistrelle bat and Soprano pipistrelle bat and from what I have learned in recent years with this developing hobby, is that they travel to a feeding site. I know we do not have any roosts in the immediate vacinity and judging by their behaviour this ties in.
Watch this space as I wait for our local bats to emerge from hibernation.