How can I identify what is the maximum frequency for iron core power inductors?
If you are in the US and unless they are knock-offs or Micrometals trademark expired recently, you can identify what material it is by the colors, yellow-white. That two tone paint scheme is Micrometals -26 material. Very common and excellent price/performance ratio. It's not high frequency nor very high flux (DC bias) but sits in the middle between which makes it a good choise for your average ~70 kHz output chokes in medium prices products.


  I can't find any datasheet


Here it is: http://www.micrometals.com/pcparts/pc_l.pdf and http://www.micrometals.com/material/index-26.html


  I don't know what is the maximum frequency


Looking at the datasheet, it starts to drop of at about 100 kHz. If your flux swing is low, you can go higher but there are better materials for very high frequencies.



  How can I measure it?


If you wind a few turns on it and have access to an LCR-meter, it will get you started for an unknown material. Critical parameters such as a loss curve is however quite time consuming to measure yourself but it is doable. The normal design procedure is to refer to the datasheet and make calculations from there.