In the name of Allah, the most Beneficent, the most Merciful.
It is obligatory to do istibraa until one is sure that no drop of urine remains in the penis after urinating.
(Maraqi Falah p.18)
With regards to you question, you should make sure that every drop of urine passes by until your heart is at ease. The essential thing is that one is free from urine before one does wudhu because anything that comes out during or after wudhu invalidates the wudhu.
After that if one fears that baseless misgivings may occur one should sprinkle water on ones undergarments. Then if there is doubt assume that it is from the wetness of the sprinkling. (Raddul Muhtar p.559 v.1)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham