Easy to check with a volt meter.
Connect either way as it doesn't matter for this method.
Connect pos lead of volt meter to pos battery terminal and connect neg lead to the battery terminal of the solenoid, get someone to crank while you watch the meter, if while cranking you get a reading above 0.5v you have a poor connection between the battery and the starter if not now connect the neg lead of the meter to the other large terminal on the solenoid that goes out to the motor itself and crank and check again, if above 0.5v the solenoids finbared.
Finally if that reading is below 0.5v then connect the pos lead of the meter to the neg of the battery and the neg meter lead to a good clean metal part of the starter body and check again and if when cranking you get a reading above 0.5v you have a bad earth and this can be checked with a single jump lead, connect one end to the neg battery terminal and the other to a good clean metal point on the engine and if it starts that comfirms a poor earth.
If all the readings are below 0.5v and your confident your battery is in good shape then your starter motor is finbared!
It's imperative that each time you take a reading the circuit must be under load(i.e holding the key in the cranking position)
Hope this helps
Chris.