McCarthy was among those in attendance at Griffin Park in midweek as Brentford saw off Southend in an FA Cup third round replay.
It is believed that he was there to cast an eye over highly-rated midfielder Forrester.
A number of clubs have been closely monitoring the 22-year-old this season, with his standout displays in League One attracting plenty of interest.
McCarthy has been impressed by the former Aston Villa trainee and may be tempted to make an official approach during the winter window.
Forrester's deal at Brentford is due to expire in the summer and he could be acquired at this stage were any interested party to meet a £350,000 rumoured buyout clause in his contract.