Rabbis collect money and donations for all types of causes and charities. How do we know that the money is used properly and for the purpose for which it's collected?
Your concern is certainly valid.
For the record, not every person collecting money who looks like a rabbi is one. Some may be scholars or rabbis, but most are regular observant Jews collecting for either private or communal needs.
In any case, we should assume that these people are, as their appearance suggests, G-d-fearing people, who are honest about their need and purpose for collecting.
Still, we are also required to be responsible in our giving, and when it comes to donating significant sums we should really only give to well-known and reliable organizations or individuals, or ascertain that the solicitor has an authentic recommendation from such a rabbi or organization.
Although it's natural to have these suspicions, particularly when we're "expected" to assume impeccable piety, nevertheless, unless there's some special reason for mistrust, we must really banish these thoughts from our minds.
In fact, this inclination is so strong that people even suspected Moshe Rabeinu of embezzling and appropriating funds and wealth donated for the Sanctuary for his own personal wealth!
The Talmud (Berachot 18b) relates how the father of the great rabbi named Shmuel collected money for orphans. Shmuel was not present when his father died, such that the whereabouts of the money was unknown and people came to refer to Shmuel as "the son of he who stole from the orphans". The soul of Shmuel's father appeared to him and revealed that the money was under the grinding-mill, saying that the upper and lower amounts were their personal monies, while the orphan money was in the middle.
When Shmuel asked why he placed the money this way, his father replied that he put his money on top so that in case a thief should find the cache, his own money would be stolen and not that of the orphans. Similarly, if the earth should damage the money, his own wealth would be damaged while protecting that of the orphans.
So we see that far from people's false suspicions, not only do the righteous not misappropriate others' funds, they actually expose themselves to losing their own money in order to guard and protect that which was entrusted to them by others!