I use a digital refractometer which gives a readout you can see; 2 popular brands are Milwaukee and Hanah. A conductivity/ salinity monitor can also give you a read out in mv which you can convert to sg or ppt. They are in the $100 to $150 range pricewise. I use this one:
http://www.testequipmentdepot.com/products.htm?item=MA887&ref=gbase&gclid=COCpmdT74r4CFYlFMgodDHsAwQ
http://www.testequipmentdepot.com/products.htm?item=MA887&ref=gbase&gclid=COCpmdT74r4CFYlFMgodDHsAwQ