how to check earth connection
I think you are speaking about the Heart connection on the mains supply line.
If yes, witout a specific instrument, try this, but remember, the operation contain a risk.
1) I assume Your mains is Line+Neutral.
2) I assume the Neutral is "good enough" to have a potential near to 0 V and enough stable during the time.
3) I assume that a some kind of GND contact on the socket is REALLY connected to the Heart, also if with a bad conection. THIS IS FUNDAMENTAL FOR SEFTY !
4) Disable an eventual "differential" circuit breaker (or connect measurement before it).
5) connect AC voltmeter between GND and Neutral (on the socket).
6) ADVISE EVERY PEOPLE INSIDE YOUR BUILDING!!
The following operation is RISK of FOLGORATION !
7) connetc a power load (like a 100W lamp) between Line and GND. Lamp should turn on, read the ACVoltmeter (V1) and DISCONNECT SOON!
8) Applying the Ohm law, the GND resistance is Rgnd=V1/(Power_lamp/V_mains)
eg:
V1= 6V
Power_lamp=100W
V_mains=220V
Rgnd= 6/(100/220)=8.2 Ohm, that is good for most applications.