Nowhere is this more true than when the specter of divorce looms on the horizon.  Even if that's not what you ultimately want, there's no guarantee your spouse is going to break down and try and work things out when you confront them.  They might deny the whole affair, make themselves scarce for a few days, or go and get a divorce lawyer.

And a divorce lawyer is the exact person you should see before you confront your adulterous partner!  Even if you think that your spouse, when confronted with their infidelity, will own up and try and work things out with you, it's still safe/good to have.  Your spouse already is doing something surprising by cheating on you.  He or she may surprise you even further by saying they want a divorce.

This doesn't mean you have to slap down a divorce lawyer's full court fee.  You should probably be able to get by with a consultation visit.  In fact, I highly recommended it--you'll probably get some good advice about how to handle the situation.

You'll also better understand the legalities of your situation.  Let your spouse know this, and they will know you mean business.  They'll also likely be more compliant--especially if you let them know when you also tell them you know they've been cheating. 

This is a double-whammy that can really catch them off guard and leave them at your mercy.  Your spouse will also be less likely to pull anything too crazy on you, once he or she understands that you mean business and know how to take care of that business as well.

This will also help keep you from being caught off-guard leally.  If divorce is the only option (you may decide this after talking to your spouse, or you might have decided it already), going to a lawyer before you talk to your spouse may well leave you with an advantage in court--before you ever even go to court!

Get your ducks in a row before court is even mentioned, and you're more likely to get what you want--or at least more of what you want, if you're male--when court time comes.  You'll be a lot more likely to avoid what could be a long, drawn-out court battle if you consult a lawyer before you spring your knowledge on your spouse.