Vortime
You have a PM.
I think you mean
VMC on top.
In general it's hard to forecast cloud tops because they are the result of processes which can't be precisely measured or modelled. Whereas bases can be got from TAFs/METARs usually.
The UK Met Office (very kindly, in the interests of safety of course
) does not release its 3D model data; they sell it to commercial weather providers who would be most unhappy if it appeared on some free website.
There are two ways I know of: one is the US-run GFS model which can be accessed via many websites e.g.
http://www.arl.noaa.gov/ready/cmet.html
and which had forecast skew-t diagrams somewhere, and the other is a Univ of Basel site I use
http://pages.unibas.ch/geo/mcr/3d/meteo/
which, under Animated Soundings, has a tephigram which is a similar kind of thing.
IFR flight planning involves working out a route acceptable to Eurocontrol (CFMU) at levels which your plane can fly at, and oxygen requirements come into it too.
I normally stick in a flight plan at FL100-130 if the weather is nice and one can always stop climb early (routing restrictions excepted) or ask for a further climb (never been refused). Normally I go for a level at least 3000ft above any tops forecast. You don't want to sit in IMC at -5C
Icing is a major issue and must be avoided as far as possible in the planning stage.
There is no easy method. You need to know where (on the internet) you can get weather data, how to make sense of it, and you need a capable plane to get above the clouds. In Europe, most airway MEAs place you either bang in the cloud or just above it, rarely below it, and this is true all year round, and you can pick up ice any time of the year.
The above is a very short answer to a very short question which deserves a very long answer!