Could you give us a few examples you've encountered ? :)
Most of the time, there's a slight nuance between those two, and they are easily confused by natives. (So, if you make such a mistake, it's not always terrible.)
How to distinguish it : Who does what.
"Je vous laisse à penser..." means "I will let you judge" (I leave it to you for judgement) ; to be honest, I haven't come across this one very often.
"Tout (me) laisse penser que... means "Everything leads to think that..." (Everything leads me to believe that...) This construction is often used for both meanings.
But, yes, if you want to be precise, here's how it looks like.
(Sth) laisse penser --> JE ---> laisser à penser (sb else)
Look it up here for more precise information : http://www.cnrtl.fr/definition/laisser (B. − Qqn/qqc. laisse qqc. à faire à qqn/à qqc.)
Hope this helps :)