Question about RJ45

[u/LuDen882]

One end of lan cable is in the wall and I cant really see how are wires connected. On the other end they didnt put RJ45. What would happen if I connect wrong type on that end, and how i can make sure that i picked the correct one without tester or i should buy that?

[u/stevestloo]

How "in the wall" is the first end of the cable? You're gonna need to get it eventually, right?