An amp meter should read all battery current except the starter motor - this is around 100A and can damage a meter that goes up to +/- 20 or 30 amps. Some bikes this is easy, there's an huge wire going from battery + to the solenoid and another smaller one (still pretty big looking, but it has an extra insulating layer) from the battery + to the harness: this carries both load and charging power. Some bikes have this smaller wire attached at the solenoid lug where that huge wire from the battery connects. I have seen bikes with the harness (load) connected to the solenoid and the charging circuit connected to the battery.
You have to combine all non-starter battery wires together into one wire and then interrupt it, connect your amp meter across the interruption with fairly heavy wire. 14ga would be good, 16ga would be sufficient, anything smaller would be a poor choice. You must fuse the battery side wire to your meter (use 15A or 20A) very close to the battery or solenoid: otherwise this will carry unfused battery power and may (will) cause a fire if it shorts out to ground!
Running these wires up to the instruments or bucket is risky, constant flexing at the steering head can cause a wire to break or chafe. If you do this use high-flex wire (a lot - a hundred or more - of very thin conductor strands twisted together) not normal car hookup wire (usually 17 strands). Add a protective sheath of some sort at the steering head where it flexes.
Using a shunt meter is best but they are rarer and more expensive. This type uses a shunt, basically a very low value resistor, in the measured circuit. Current through this resistor produces a voltage across the resistor. The meter measures this voltage. Usually shunts are rated at 50mV for the rated current: a 100A shunt will make 50mV at 100A. You need to match the meter and shunt rating to get accurate readings. The wires from the shunt to the meter can be quite small, but they need to be fused near the shunt with a low value fuse (1A or less) as they will have +12V battery voltage on them and will be otherwise unfused. If you mount the shunt safely, run the power wires carefully, and dress them securely (tied down well) then you can skip fusing the shunt circuit. A fuse is still fine but if the shunt wiring can be done as safely and securely as the fuse wiring would be, it's about the same risk.
You can measure battery volts if you use a relay powered by ignition power to switch the meter feed connected directly (through a fuse!) to the battery. This is a pretty good idea if you are monitoring the charge level on the battery since ignition circuit voltage is always less than battery voltage and changes depending on where in the harness you measure it.
