OP, why would you have to take your earrings out to pummel him? I'd probably leave my earrings in so I'd still look good after I was finished.
OK, let's recap.
DH got an inappropriate text.
DW got some "insider" information that DH is being a dog at the gym.
DW tipped her hand a bit by letting DH know she saw the inappropriate text.
So what to do now...
Prepare & Protect yourself first.
I would take a look at and copy all my financial information and put it in a safe place outside of my home. Make sure you get tax returns, retirement information, savings and checking accounts and copies of all credit card accounts. Make you sure date them all so that you know on "X" date you had this much money, owed this much money etc.
Get any jewelry of any monetary value out of the house (good for hocking later if necessary).
Get any small items (things your husband wouldn't notice/miss) with any sentimental value out of the house.
Open a savings account (preferably in the name of someone else who you trust) and start putting $ into it.
Keep an eye on credit card usage in the future, to make sure big bills aren't being run up.
Keep an eye on all savings/checking/retirement accounts in the future to make sure money isn't being withdrawn.
Perhaps your stepmother would be willing to allow you to store the items at her house & open an account using her name. If not her, then perhaps a trusted friend would be willing to help you.
Now there are those who think that these steps are a bit premature, but it has been my experience when my friends have been in these situations that they decline quickly. None of this is bad if the OP works this out. It's not bad for the OP to have a handle on her financial situation. It's not bad for the OP to have control over her jewelry/sentimental items & if the situation rights itself the jewelry & other items can always come back into the home. It's not bad to have a savings account and if the situation rights itself that money can always be placed into the joint accounts at a later date.
The next thing I would do is to start getting cell phone logs & see what cell phone usage there is.. who he is calling and how frequently.
I wouldn't say anything about knowing someone at the gym. That's not information he needs to know and you might need to ask that person for some assistance. The last thing you want to do is have husband know everything you have in your arsenal while he's sneaking around. He has no problem keeping things from you so you should, at this point, play things close to the vest as well. Chances are he's in panic mode right now figuring how to go further underground with this, so you can probably get all the stuff I suggested done without him realizing it's happening.
I'd also probably talk to an attorney just to see what my options are in terms of separation and divorce.
Then I'd watch and wait. I know the intial reaction is to pummel him, scream, tell him everything you know etc. But you have to think rationally and logically so that you protect your own interests.