How to know, just by looking at the formula that it is a strong or weak field ligand?

dear payal 
here is the trick , for most of the ligand it works , but not for all , but it can satisfy your demand for entrance exam questions . 
1- the ligand which contains carbon and nitrogen or starts with C AND N , then its strong field ligand . 
2- the ligand which starts with O and H , and if it contains oxygen and hydrogen then its weak field ligand . 
HOPE IT CLEARS 
regards 

at first you have to see the lone pair donating tendencyof the ligands.if the ligand contains electronageative legands and owns one or moe than one lone pair it is styrong filled and if th ligans contaion such atoms which is too electronegative to donate lone pairs will be considered as weak filled ligands