You can use pH paper (litmus paper) or you can go to just about any nursery or big box store that sells plants and find a pH meter for under 30 dollars US, either of these will give you a good
enough reading.
It is possible to "eye ball" pH but most likely you will be off just enough for plants to notice.
If you need to increase alkalinity (above 7.0 which is neutral) then you can use crushed up gypsum board usually easy to find where drywall is going up at construction sites, or lots of ground (crushed) sea shell or egg shells but it will take a lot of those two.
Or if you can find it agricultural lime.
If you need to decrease alkalinity (below 7.0) then you can use ground sulfur, or even diluted battery acid (which is sulfuric acid) will work when used very carefully as a
water in amendment.