Here's how to view the GSM signal value on your smartphone:
When your phone connects to your operator's network, it searches for the nearest antenna and tries to connect. To do this, the mobile phone analyzes the received signal and adjusts its emission strength (in addition to other parameters) in order to reach the transmitting antenna.
In order to choose a GSM repeater (also called a signal amplifier) wisely, it is very important to first measure the GSM signal.
1. With an iPhone
First of all, to measure the 4G signal, you have to move to where you intend to install the external antenna (the higher you go, the better the received signal will be) and after having deactivated (if active) the wi-fi and bluetooth, simply type *3001#12345#* on the iPhone keypad, then press the call key 
:a menu (Field Test) will open where you have to click on the LTE (or 4G) item. Then click on Serving cell meas: locate the item rsrp0 which indicates the intensity of the 4G signal.
What interests us is the value corresponding to the rsrp0 item: it is the value that indicates the quality of the signal received by your iPhone in that precise geographical position.
2. With an Android smartphone
If you need to measure the 2G call signal, first you have to switch to 2G mode in this way: go to the phone settings, click on Network and Internet, then on Mobile network, then click on Advanced and finally on Preferred network type . Then choose the 2G (or EGSM) voice.
Move to where you want to install the external antenna (the higher you go, the better the signal quality) and, after deactivating any wi-fi and bluetooth, go to your smartphone settings 
, at the bottom of the list, choose the About device/phone item. In the following screen click on Status (Sim Status or Network). At this point you can read the level of the received signal, under the heading "Signal strength":
Obviously you can return to the 4G generation at the same time as you went to the 2G generation, with the difference of having to click on 4G (or LTE) instead of 2G.
Depending on the version of Android you have installed on your device, you can still dial *#*#4636#*#* on the keypad, then press the call key 
:you will get the same information.
We can rightly say that our mobile phone is a "field meter", certainly not advanced, but perfectly useful for the purpose.
To measure the 4G mobile signal, the procedure is different: first download the Netmonitor Cell Signal Logging app (icon similar to a clover, written LITE in the middle) and, always after moving to where you want to install the external antenna, with bluetooth and wi-fi disabled, open the application. The value corresponding to the intensity of the signal is found in the main page (Main), at the top left, in correspondence with the RSRP item.
Now you can move around and do some tests to find the points where the signal reaches the best. Speaking of dBm, the best value is a negative number as close to zero as possible (eg -75 dBm is better than -98 dBm). Be patient: the phone takes a few seconds to make these measurements.
If it can be useful, also consult these two blogs How do I measure the mobile signal with Android? and How do I measure the mobile signal with iPhone?. Good measurement!
