The USPTO contains all federally registered trademarks as well as a list of marks that applied for protection but were denied. Searching this database can certainly turn up what seems like an immeasurable number of results. The key is to knowing how to properly and effectively limit the preliminary trademark search from results in the millions, down to results in the twenties and even down to single digits. Getting a manageable list of results makes it much easier to identify the trademarks that are relevant to the proposed trademark.
Counsel needs to evaluate each relevant mark in order to determine whether any mark is confusingly similar to the proposed mark. Factors that must be considered include: similarities of sound, appearance, meaning, and the trade channels that are used for the registered trademark and the proposed trademark. After evaluating the results, counsel will draft or present his opinion as to if the proposed mark is likely to infringe on a previously registered mark and prevent the successful application of the proposed mark.