If one has prepared and inspected oneself properly before immersion, there is no need to look for problems afterwards. However, if a woman is concerned that her tevilah may not have been valid, she should not hesitate to ask a question.
According to many authorities, one should not shower again at the mikveh after immersion. This ensures that one doesn't imagine that the shower, rather than the mikveh, made one tehorah.
When a woman comes home from the mikveh, she should tell her husband verbally that she immersed.
Finding a Problem after Immersion
If one has prepared and inspected oneself properly before immersion, there is no need to look for problems afterwards. In fact, it is best to avoid looking oneself over after one has immersed.
If, before leaving the mikveh, a woman finds a chatzitzah or realizes that she omitted an important element of the preparations, she should call the mikveh attendant and ask her what to do. The attendant may advise removing the chatzitzah and immersing again, possibly without making a bracha. If one feels uncomfortable, it is always acceptable to request to immerse again without a bracha, or to ask the attendant to call a rabbi for a halachic ruling.
If a woman finds something after she has already returned home, a second immersion may still be necessary. It is important to consult a rabbi or yoetzet as soon as possible. The answer will depend on how serious the problem was and on when it was discovered. Generally, rulings are more lenient if a woman has already come home from the mikveh, and especially if the couple have had relations.
There are a few possibilities for asking a halachic question late at night. It is worth calling the mikveh if it is still open; many mikvaot have a rabbi "on-call" to answer these types of questions. Nishmat's Golda Koschitzky Women's Halachic Hotline also operates until midnight Israel time.