Adding an ammeter to a small workshop
Building a small panel with a panel mounted meter would be doable but it can be more work than meets they eye, you have to cut the cover for the meter, and you need a lot of room inside - the terminal studs on the back of the meter are usually pretty long and uninsulated.  

An alternative would be to use a good heavy duty plug strip with a 12A breaker in the strip.  This way with any luck the breaker on the power strip would trip before the breaker in the panel.  You could use a Kill-a-watt with the power strip to get an idea what kind of load your tools demand.  

If someone in the other room plugs in a space heater or hair dryer when you're already drawing a few amps, there's not much you can do from your end to avoid tripping the breaker.