To try resolutions only, you can resize the browser window, but for devices, there are not a lot of ways indeed...I think the best not so expensive idea would be to ask for feedback in the work in progress or ask some people to try the game on their side, either on the internet or scirra forums, or in real life (in real life, you have the potzntial advantage of seing it yourself).
I always though of having a test game group with people willing to help, but then I though that the idea was not so great since I am not sure people could help on a regular basis for free.