When you have doubt you should consider is as pure.
Generally, when something was initially pure but there is doubt about it becoming Najis, it should still be considered pure. And it should be considered najis only if you are sure it is najis. and wneh somethins was initially Najis and you have doubt about it becoming pure, it should still be considered najis.
Also, it is recommended for men to perform the action of Istibra.
Istibra Issue 73: Istibra is a recommended act for men after
urinating. Its object is to ensure that no more urine is left in the
urethra.
There are certain ways of performing Istibra, and the best of them is
that after the passing of urine, if the anus also becomes najis it is
made Clean (tahir/pak) first. Thereafter, the part between the anus
and the root of penis should be pressed thrice, with the mid dle
finger of the left hand. Then the thumb is placed on the penis, and
the forefinger below it pressing three times up to the point of
circumcision, then the front part of the penis should be jerked three
times.
Issue 74: The moisture which is discharged by man during wooing and
courtship, is called 'Mazi'. It is Clean (tahir/pak), and so is the
liquid which is seen after ejaculation. It is called 'Wazi'.
Similarly, the liquid which at times comes out after urine, is called
'Wadi' and it is Clean (tahir/pak) if urine has not reached it. If a
person performs Istibra after urinating, and then discharges liquid
doubting whether it is urine, or one of the above mentioned three
liquids, that liquid is Clean (tahir/pak).
Issue 75: If a person doubts whether he has performed Istibra or not,
and then discharges a liquid about which he is not sure whether it is
Clean (tahir/pak) or not, that liquid is will be deemed najis, and if
he has performed Wudhu it becomes void. However, if he doubts whe ther
he performed the Istibra correctly or not, and a liquid is discharged
about which he is not sure whether it is Clean (tahir/pak) or not,
that liquid will be Clean (tahir/pak), and it will not invalidate the
Wudhu.
Issue 76: * If a person performs Istibra, and also performs Wudhu, and
if after Wudhu he sees a liquid discharged, of which he knows that it
is either urine or semen, it will be obligatory upon him to do Ghusl,
together with Wudhu. But if he had not done Wudh u after Istibra, then
Wudhu alone will be sufficient.
Issue 77: When enough time has lapsed since urinating, and one becomes
sure that no urine is left in urinary passage, and then he sees some
liquid, doubting whether it is Clean (tahir/pak) or not, he will
consider it as Clean (tahir/pak), even if he had not done Istibra. If
he has Wudhu, it will be valid.
Issue 78: Istibra is not meant for women, and if she sees any liquid
and she doubts whether it is urine, that liquid is Clean (tahir/pak),
and it will not invalidate Wudhu and Ghusl.
Reference