A sensible course of action would be to ask for some references from other organisations that the company has worked with. It would also make sense to check out the details that the organisation gives you, for example, phone the telephone numbers and look over the website.
Warning signs that an organisation might not be reputable include ignorance of charity law or badly planned fundraising proposals that do not constitute good practice.
Membership of the Institute, either by an individual or by an organisation, means that members agree to abide by the Codes of Fundraising Practice and should indicate a reputable firm.
If the organisation is engaging in fraudulent practice, then the Charity Commission might have already heard of them and you could try checking with them.
