To add to some already great answers here, the very first thing you should do is listen on the repeater's input frequency.
Do you hear it then? If not, then drive fairly close to the repeater site. Note that an off-frequency signal that overloads the repeater's receiver might also overload your HT. If that happens, then it would be prudent to attenuate the signal to determine if it suddenly disappears at some point or not.
The best way to do that would be to use a step attenuator between your antenna and transceiver, but it could also be done by gradually covering the antenna with aluminium foil*. Or, you could simply drive a little farther away from the repeater.
If the signal suddenly disappears at some point as the attenuation is gradually increased, then it likely is only a spurious "phantom" signal due to the input preamps overloading, and not malicious QRM.
You can examine those signals using a spectrum analyzer. It could also be a product of two signals mixing. But I'll leave the details of that for someone else to expound upon.
Only after doing this should you proceed to triangulate the interference source, assuming that is feasible.
*Don't transmit with your HT's antenna covered! It might damage your HT's output circuit.