need a pic m8, of the browning, could be overwatered could be something completely different. As for testing the soil PH. You can get a cheap PH soil testing kit from your gardening store, they're very easy to use and come with instructions.
You basically mix the soil with some water in say a glass or something transparrent then you add the PH chemical to the water/soil mix and match up the colour with the PH chart that comes with the kit. You should be able to pick a kit up for a few bucks/euros/pounds.
Another tip buddy, I wouldn't use MG soil on your freshly germinated seeds as it has slow releasing nutes in it that last for 3 months which i've found burns the plant like fuck. If it is outside then the rainwater will be full of everything the plant needs. I've been in your position before with that damned MG soil.
If your on a tight budget (like me) then get some multi-purpose soil and re-pot as soon as you can and stay away from MG.
If you can show us a pic we could help you further.
Take is easy braw.