There is... but it's not very ahhh, "legit".
It also depends on how many amps you anticipate the slot cars will draw.
You can wire a 1R 10W resistor on the positive power rail, and measure the voltage across it.
V = I*R
I = V/R as R = 1, V = I.
You are able to set the range of current you wish to measure by using 10R, 100R, 1k, 10k, 100k etc. etc.
The problem with this can be, the resistor may dissipate more than 10W and thus fry itself. Make sure you check to see what the maximum current may be, and apply the formula P=I^2*R, to see how much power the resistor will be dissipating at maximum current.
I stress that you take care in ensuring that you choose the right value resistor with the correct power rating, and make sure the resistor you choose wont drop more than 5V (Or whatever the rating of your panel meter is), otherwise you may cook your panel meter as well.