For the "main" forks like BCH, BTG, BTCP etc... you can simply use their fork of Electrum... and open a copy of the wallet file with that software:

ElectronCash (BCH): https://electroncash.org/#download
ElectrumG (BTG): https://github.com/BTCGPU/electrum/releases/tag/3.2-beta
Bitcoin Private Electrum (BTCP): https://github.com/BTCPrivate/electrum-btcp/releases/
Bitcoin Diamond (BTCD): https://github.com/eveybcd/electrum/releases

NOTE: I do NOT endorse any of these wallets... and make no claims as to the "safety" of any of them.

As always, you should make sure that all your BTC has been safely moved to a new wallet before attempting any fork claiming etc.