All donations to the Tory party are in the public domain so if you're really that interested then it's easy to find out. I think that the presenter on 5 live this morning said that 11 Russian oligarchs had donated to the Tories.
And, by law, all donations of over £500 are from UK residents or UK based companies. It won't stop someone creating a UK business to channel funding through but it will be pretty obvious, and a weapon that could be used by other parties.
The Electoral Commission scrutinises the accounts of every party and, although these allegations are made at each election, there's never been any proven. Lets wait and see...