It is comprehensible that you like know your donation goes not in vain and the one accepting your donation is legitimate. This is because you like please that your donation goes well and effective so that you will multiply good Karma when you rethink and think about how it is well contributed. The most important thing here is you need to multiply your pleasant feeling by thinking again and agin when you did a good, legitimate donation.
So for donation, it is wise to choose people to whom you donate. If you cannot choose (are you doing donation on regular basic and difficult to examine/check at the point of time of donation?) try not have second thought, regret, disappointment after donation. At least, try to please by reviewing yourself that you got rid of attachment on things/money that you just donated. You should not linger your thoughts about quality of donee(s) which could do harm on your pleasantness of doing donation (difficult and challenging mind exercise for most people).
To inculcate Metta, you need to know it is opposite of aversion and hatred. Our Lord Buddha said many times that Metta is unmeasurable, limitless making up of your mind. So Metta is not correlated with quality/value/ethic/intention or any physical quality or moral status of any living being you are trying to spread on. It is not appropriate just spreading Metta to good people or people good to you. You can put Metta to any living great benefactor as well as living tyrant dictator. To achieve this you have to understand that no matter the target living being shall receive your Metta or not, you definitely have peace and pleasant mind without aversion and hatred by spreading Metta to infinite living beings around all directions (east, west, south, north, ++, up and down).
By spreading Metta to all living beings(unlimited), you should expect the following effects of it (need citation in Pali Canon).
- You can sleep soundly.
- You can wake up gently.
- You do not have bad dream.
- You can be protected by Gods(Devas).
- People loves you.
- Gods(Devas) love you.
- Weapons, fire and poisons cannot harm you.
- Your face is cheerful.
- You have elegant/respectable moral esteem.
- You have strong Sati till you would have last breath.
- If you cannot do leverage on your Metta Jhana to Vipasana, you can still be Brahma (rebirth at one of Brahma realms after life).