How can I tell if a particular multimeter measures AC with RMS or Average
True RMS measurements is a property the manufacturer will be very proud about to share with you as customer. It is usually printed in bold on the meter itself. True RMS isn't printed on the meter in your example, so it won't be one.

Also I own a more expensive brother of this meter and on that one it says 'True RMS' (in bold print).

Third way to tell is from the specifications. Again the manufacturer will be proud to be able to share True RMS properties and I expect it listed high in the 'key features' and next to the 'ACV' and 'ACA' table headers.

In conclusion: No this is not a True RMS meter.