Design of low current sensor from 50mA to 2A in a dc-dc buck converter
Yes, you can use a small current-sense resistor to get a voltage proportional to current.

However, consider the competing tradeoffs.  You want this to drop as little voltage as possible to not effect the system being measure.  However, the lower the voltage, the smaller it is compared to the inevitable noise.  If you are measuring the current out of a 15 V unregulated supply, then 500 mV isn't going to matter much.  However, if you are measuring the current out of a 4 V battery, then even 100 mV drop can degrade performance or lifetime.

You can put the sense resistor in series with the current, then use a diff amp to get a ground-referenced voltage proportional to the current.  However, it's easier to use a chip specifically intended for high side current sensing.  Check out the INA139 and INA169 from Texas Instruments, for example.