Here's an excellent video on how to bench test your starter relay. The starter relay is often the culprit when encountering starting issues with an old motorcycle, especially if it's been idle for a long period of time. 
Keep in mind when replacing a starter solenoid, that there are many poor quality after-market replacements on the market, so original equipment is always best if you can find one.
Here's a link to our selection: https://www.hoopriderparts.com/category/starter%20solenoids