My 1st step would be to check voltage at the starter's 'fat' terminal(s), while cranking. To get valid results, you need the negative probe directly on the battery's negative post (not the terminal; the post). The positive probe should be directly on the starter's positive post. Note the voltage. Move the positive probe to the starter's frame (ground), and test again. Last, move the positive probe to the battery's positive post (not the terminal; the post). Repeat the test and note the voltage.
All voltages below assume that you're using an SLA battery (Odyssey, etc) in good condition. Lithium tech batteries will likely show significantly higher numbers.
The 1st & 3rd test voltages should agree within ~0.5Volts, and should be at least ~8.5--9Volts, up to a max of around 10.5Volts. The 2nd test should read at less than 0.5Volts.
It can be tricky to be sure you've made good contact with both probes, so be sure the probe makes it through any corrosion (which may not be obvious to the naked eye).
If your battery voltage stays around ~8.5-10.5V while cranking, and you see a significant deviation from the other numbers, you may well have higher than normal resistance somewhere in the start circuit. If voltage at the battery terminals stays at or near the 'resting' voltage of the battery, you almost certainly have high resistance in the start circuit.
Hope that's of some use,
Charlie