Today's legal market is highly competitive, and though lateral moves are currently common, with few people working their whole lives at one firm, finding time to discover what positions exist, and doing so covertly so as not to alarm your current firm, may prove challenging, especially in respect to your work load. Meanwhile, legal recruiters keep making cold-calls. Should you screen them? Field them, even if you aren't interested in switching: you will at least gain insight into what you are worth on the market, and that grants leverage with your own firm.
Legal recruiters offer free information about the job market, and only ask for a few minutes of your time. Taking some of these calls might enlighten you. Nevertheless, you want to be smart, and that means knowing who are dealing with.
Ask a lot of direct questions regarding the recruiter's expertise. You want to know whether he specializes in your area of practice, and what specific locations he prefers to work with. Vague answers such as that he works "nationally" or represents many areas of interest are bad signs: he may be inept and may misrepresent. Worse, he may pillage your résumé and use it to gain some résumé quota, meanwhile ineffectually plastering your name across the board of major firms.
Therefore be smart, be choosy, and withhold giving your résumé too soon. What you want, first of all, is to establish a bond of trust. There are enough legal recruiters for you to be choosy. Do you feel right about this person? Don't trust your gut alone. Research him intensely, if you are considering working with him, by looking at his company's website, his personal bio, his LinkedIn account, and contacting his former clients. Further still, ask for references, and follow up with them. Also ask around, and hear who has some repute and good standing in the field.
Avoid the misuse of your résumé, as already mentioned, avoid recruiters who offer you jobs that don't exist, only to get your résumé. Check his background before you give him anything like a résumé.
Once you've decided to work with a recruiter, set a specific agenda with him, working out a clear tactic that will get you what you want without distracting you from your current work and without alerting your firm to your intentions.
Taking care to discover who you are working with ensures you will find somebody worthwhile. In the meantime, listening to a pitch or two from different recruiters, and asking a lot of questions, can give you a sense for the field and your standing therein. Be willing at times to give them an ear, but be careful in who you disclose information to. You will want to know how many interviews and placements a recruiter's firm has secured; you will want to know who you are working with. Having gained this, a recruiter can take the pain of searching out prospects for your lateral move much easier. Gaining the best position is easier when working with a legal recruiter who knows what they are doing.