Honestly, anyone who can say that they aren't in the pockets of anyone, should be able to prove it, so if they are going to take money for the race, than they should where the logos of their supporters, like Nascar drivers do, this will be a way to make sure that we know who is buying our elections.
Politicians should be required to disclose information about their donors. Nowadays, although there are limits on money from donors, the people deserve the right to know the identity of politicians' donors. This information helps the people get a better understanding of a politicians' backing and support group, which will help establish a more educated vote.
Yes, politicians should reveal their donors before they give a speech. The biggest donors are from wealthy businessmen. The amount of donations are usually disclosed. If there is some reliance on secretly funded non-profits then there could be ambiguity on how much money is being spent overall. The public will distrust politicians The public will voice their concerns on social media.
No, politicians should not be required to reveal their donors before giving a speech. According to first amendment rights of every United States citizen, there should not be prohibition to freedom of speech and/or freedom of assembly and petition. Whether or not a politician reveals his or her donor is irrelevant with the right to speak about a public matter that concerns US citizens.
In some cases, it can be helpful to have politicians state who they're supporting because they've been supported by them. But not before every speech. This would be a waste of time and productivity, even though I get the intention behind this question, and it is a good, noble one.