How do you know the occurrence loop if there are in a network switch of the type is not Cisco? How to avoid loop networks?

i suggest by unplugging segments of the network from the switches and thus narrowing down where it's, and to avoid it you can use any of the STP (Spanning Tree Protocol) Flavors it's an open standard protocol not closed to cisco only.

If you see the port which the cause of the loop you will see it blinking fastly leading to lightening constantly and you should trace after that the physical cable which caused that problem

if you use monitoring tool or program it will simplify that

باختصار هناك بعض سويتشات من شركات غير سيسكو تكون غير مفعل عليها بروتوكول STP الذي يحمي الشبكه من LOOP وتجد البورت المتصل يضيئ وينطفئ بشكل متناسق وهذا دليل على حدوث loop بالشبكه

as Mohammad Elsayed said it is correct when the loop occur in L2 Switches the blinking of led light of that the port happen very very fast. and to stop use STP supported switches like cisco, HP Pro curve or other verder to avoid looping occured on L2 Switch.