Search in an Arabic dictionary (masdar)
Go to Almaany's Arabic-ِArabic dictionary. Enter the word for which you want to look up the masdar, for example ذَهَبَ and click on 'ابحث' (search). Look at al-fi'l (2nd result) and not at al-ism (1st result). First you will see the verb conjugation, then the masdar. The verb ذَهَبَ has two: ذَهَابًا, ذُهُوبًا.