Ask your boss if the company is an equal opportunities employer. The boss will almost certainly say yes. Then ask if this equality extends to pay. If you're still not happy go in again with your union rep or a senior female colleague and ask again. Don't be fobbed off. Go for it, you've earned it!
Your employer could claim - and probably be right - that the pay is based on experience, quality of work, length of time with the company, depth of commercial experience, training, qualifications, and so on and so forth. The pay structure could have nothing at all to do with your gender or the gender of your collegues. And no, there is no way to find out other than asking those you work with what they earn, taking the HR lady out to get sloshed or riffling through their rubbish bins.