Credit personnel in every industry are besieged almost daily by representatives of collection firms across the country. While this may be a constant annoyance, the fact remains that the services which are offered by a professional collection firm are a must for any company extending credit. The following suggestions are offered as an aid to assist you in quickly weeding through the numerous solicitations you receive to select the firm which best suits your needs and exemplifies your company's policies:   1. KNOWLEDGE OF YOUR INDUSTRY - do not hesitate to question the firm to whom you are considering turning over your delinquent receivables about their experience within your industry. The best indication of that firms ability to service your accounts is the extent of their experience. Ask for a list of industry references and talk to several people to make sure they are pleased with the service.   2. INTEGRITY - A) Request information on the backgrounds of the principals of the firm. In contacting references find out how long they have been doing business with the firm and/or principals. If practical, request to meet with a member of the firm personally. B) Determine the length of time monies are held on deposit in escrow in the firm's special client account before remittance. Make sure that monies collected are maintained in a separate account.   3. PROFESSIONALISM - when turning an account over to a collection service, remember that they are acting as an extension of your firm. As a result, you should make sure that their practices as your agent are ones with which you are comfortable. If you like, ask for a sampling of dunning notices which are sent and a presentation of telephone demands.