You could check the battery cable connections especially the ground cable at the block but... your best bet is buy a starter.
The clowns at part stores cant check the starter properly. My nephew took his starter off and put it back 3 times because 3 different part stores said it was good. When he called me I took the starter off again for him and stuck the jumper cables to it right there in front of the Exploder. The starter ran. He says, "So its good" I said "No its not. This little bugger should start with so much torque it would nearly roll itself over. What we see here is a like fan motor, not a engine turner'. New starter, end of story.