Harbor Freight has a digital angle guage for $30, maybe even cheaper on sale in their stores. http://www.harborfreight.com/digital-angle-gauge-95998.html
I have the $5 model from HF. Works well for me but I'm not building fine furniture. http://www.harborfreight.com/dial-gauge-angle-finder-34214.html
For 90 degrees on my scroll saw I just use a piece of 2x4, cut, turn over, cut, check. Cut twice, measure once?