How to avoid getting trapped on top VFR?
By regulation you just have to be sure that the conditions at destination allow you to maintain VFR clearance from cloud for your descent.  Pretty much all you have to go on is the TAF for the area of your destination.  If there isn't a TAF for an airport within say 20 or 30 miles from destination that would be a problem for me. 

Assuming a TAF is available, you technically could go if the forecast was for broken cloud below you, hoping for a large enough hole to get down (basically a hole at least 4000 feet across, with you in the middle of it).  But that's taking a bit of a risk.  

It would be imprudent to go with anything more than scattered cloud in the TAF at any level below you at the time of your planned arrival.  And even scattered cloud in late afternoon is pushing it.  On top of that you are continuously monitoring the METAR at destination for detrimental changes, and you also have a planned alternate that has better forecast conditions than your destination.

One of the more important factors to keep in mind on a trip like that is the temperature/dew point spread at destination (which will determine the cloud base if there is enough humidity), and the time of day.  When I was bush flying in Northern Ontario many many years ago, with many hours spent scud running under low cloud, the dew point spread was the most important bit of weather information and a tighter spread was more acceptable in the morning than in the afternoon.

A morning trip, with a decent temp/dew point spread that makes unexpected cloud formation below your level unlikely with temperatures naturally destined to increase until early afternoon, is lot safer than an afternoon trip where the spread will be decreasing and more-than-forecast cloud formation at a level below you is more likely. 

Personally, I would never do VFROTT because I'd be biting my nails the whole time worrying about the engine quitting (I've had a mechanical engine failure) and being forced to descend through a cloud layer and then trying to find a landing area underneath it, if there was a decent ceiling.   

And there's the rub; you not only have to come up with good weather margins, you have to have iron clad confidence in your engine if you're doing it in a single (which lots of single-engine IFR pilots do - me not so much).